Onion Gratin

This tasty Onion Gratin flavored with rosemary and thyme was inspired by the creamed onions my mother always made for Thanksgiving. You might think of onions as a higher-carb vegetable, but I bet this is a lot lower in carbs than you think! See tips in the recipe if you want to make this ahead for a holiday meal.

PIN this delicious Onion Gratin to try it later.

This Onion Gratin is a delicious Thanksgiving side dish and it’s great for any kind of special meal! This recipe is something I worked on when I wanted something similar to creamed onions my family always had. I knew I’d have to get creative for a healthier onion dish since the old family version had flour, butter, cream, breadcrumbs, and cheese. And I ended up with a recipe that’s kind of a riff on my popular Broccoli Gratin. (That recipe is perfect for Thanksgiving if onions aren’t a winner for you!)

And recently I made some very slight changes to this recipe to make it more user-friendly and improve the results. And this time I used lower-carb yellow onions. When I recalculated nutritional information with those changes, the built-in calculator on my site and the online one where I double check showed this was much lower in carbs than the original nutrition data! So I’m updating that information with the new results. If nutrition numbers are hugely important for you, you might want to use whatever nutrition calculator you trust and re-check.

I tested this updated version of Onion Gratin on a group of friends and everyone loved it! And I think some people who aren’t normally onion fans might be won over by the cheesy topping. If you like the idea of a cheesy onion dish for a special meal, I hope you try the recipe!

What ingredients do you need?

(This is only a list of ingredients; please scroll down for complete printable recipe. Or if you use the JUMP TO RECIPE link at the top of the page, it will take you directly to the complete recipe with ingredient amounts.)

• medium yellow onions
• Olive Oil (affiliate link)
• minced fresh rosemary, or use a smaller amount of ground rosemary (affiliate link)
• minced fresh thyme leaves, or use a smaller amount of Ground Thyme (affiliate link)
• mayo
• fresh-squeezed lemon juice, I used my fresh-frozen lemon juice
• Dijon mustard (affiliate link)
• Pizza cheese blend
• Parmesan Cheese
• salt and fresh ground black pepper to taste

What is a gratin?

A Gratin is traditionally a dish where food is topped with a browned crust that contains cheese, breadcrumbs, or butter. I’ve become a huge fan of cheesy gratin dishes like this one, and I don’t think breadcrumbs are needed to make it taste good!

What kind of onions did we use?

I first tried this recipe with sweet Vidalia onions, but they’re much higher in carbs. For the lower-carb updated recipe I used medium yellow onions.

What steps add flavor to this Onion Gratin?

Please don’t skip the step of roasting the the onions with rosemary and thyme before you assemble the gratin. Then when you put them in the gratin dish, the partly-cooked onions are spread with a topping made with mayo, lemon juice, Dijon, a blend of cheeses, and Parmesan. Put that back into the oven to cook until the topping is starting to brown and you’ll have a winner.

Can you make the Onion Gratin without fresh herbs?

If you have fresh rosemary and thyme, please use them for this recipe. But you can definitely make this with 1/3 tsp. (or less) ground rosemary and 1 tsp. (or less) ground thyme if you don’t have fresh herbs. If you have whole dried rosemary and thyme, grind it in a Mortar and Pestle (affiliate link) or pound with a meat mallet.

Want to see more options for holiday gratins?

If you enter the word “Gratin” into the search bar on this website, you’ll find recipes for tasty gratins with Cauliflower, Delicata Squash, Cabbage, Turnips, Broccoli, Spaghetti Squash, Sweet Potatoes, Brussels Sprouts, and Artichoke Hearts! There are also a few main dish gratin recipes with meat and vegetables.

How low in carbs is the Onion Gratin?

If you have a moderate serving size, this tasty onion gratin has only 7 net carbs per serving with 9 grams of protein!

How to Make Onion Gratin:

(This is only a summary of the steps for the recipe; please scroll down for complete printable recipe. Or if you use the JUMP TO RECIPE link at the top of the page, it will take you directly to the complete recipe.)

1. Spray a baking sheet with nonstick spray, and preheat oven to 450F/232C.
2. Peel onions and cut the onions in thick half-inch slices. 
3. Lay the onion slices on the baking sheet.
4. Brush onions with olive oil, and sprinkle with fresh chopped rosemary and thyme. (See NOTES in the recipe if you have dried herbs.
5. Roast onions for 15 minutes. Then remove onions and turn the oven down to 375F/190C.
6. While the onions roast, mix together mayo, lemon juice, Dijon mustard, grated pizza cheese, and Parmesan cheese.
7. Transfer onions to the baking dish. Then spread the mayo-cheese mixture over the top of onions.
8. Bake the gratin about 30 minutes, or until the topping is nicely browned.
9. Season the finished dish with salt and fresh-ground black pepper to taste.
10. Then serve hot and wait for the compliments! (See notes in the recipe below if you’d like to make this ahead for a holiday meal!)

Make it for a Carb-Conscious Meal:

I think this Onion Gratin is perfect for a holiday side dish! But if you want to make it for a regular week-night meal, I’d serve with a low-carb main dish that doesn’t need the oven, like one of these:

• Air Fryer Lamb Chops
• Balsamic Pork Roast (Instant Pot or Slow Cooker)
• Chicken with Lemon, Green Olives, and Capers
• Air Fryer Salmon (with Mustard-Herb Sauce)
• Air Fryer Pork Chops

Want Low-Carb Thanksgiving Ideas?

Check out some of these recipe collections if you want ideas for keeping it low in carbs for Thanksgiving:

• Keto Vegetable Recipes for Thanksgiving
• Keto Brussels Sprouts Recipes
• Thanksgiving Side Dish Recipes (Low-Carb and Gluten-Free)
• Keto Broccoli Recipes for Thanksgiving

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4.5 Stars (21 Reviews)

Yield: 8 servings

Onion Gratin

Prep Time 15 minutes

Cook Time 45 minutes

Total Time 1 hour

I love this cheesy Onion Gratin that's flavored with rosemary and thyme, and this is a perfect side dish for a special meal! And see tips in the recipe if you want to make this ahead and reheat when you serve it!

Ingredients

• 6 medium yellow onions
• 1 T olive oil, for brushing onions
• 1 tsp. finely minced fresh rosemary (see notes)
• 1 T minced fresh thyme leaves (see notes)

Gratin Topping Ingredients:

Toggle ingredient group

• 3/4 cup mayo
• 4 tsp. fresh-squeezed lemon juice (or more)
• 3/4 tsp. Dijon mustard
• 1 1/2 cups Pizza cheese blend (see notes)
• 1/4 C grated Parmesan cheese
• salt and fresh ground black pepper to taste

Instructions

1. Spray a baking sheet with non-stick spray. Preheat oven to 450F/232C.
2. Peel onions and cut in 1/2 inch slices. I cut four slices per onion, for a total of 24 slices.
3. Arrange onion slices in a single layer on baking sheet.
4. Brush onions with oil, then sprinkle with fresh minced herbs or dried ground herbs.
5. Roast onions 15 minutes. Then take them out of oven, and turn the oven down to 375F/190C.
6. While onions roast, combine mayo, lemon juice, Dijon mustard, pizza cheese, and Parmesan cheese in a small bowl. Mix together with a fork. (The mixture will be lumpy.)
7. I used an oval 9″ x 13″ casserole dish for this recipe. I sprayed it with nonstick spray.
8. Use a turner to layer partly-cooked onions in the casserole dish.
9. Spread topping over onions (I used a rubber scraper to spread it around. Don’t worry if the mixture doesn’t cover every bit of the onions.)
10. Place casserole dish in oven and bake about 30 minutes, or until top is golden brown and onions are slightly bubbling.
11. Season the finished dish with salt and fresh-ground black pepper to taste.
12. Serve The Onion Gratin hot and enjoy!
13. When we tested this slightly improved version of the recipe, I refrigerated it overnight and reheated the next day (covered with foil) at 350F for about 30 minutes. I served it to my friends, who absolutely loved it.

Notes

You can use white, yellow, or Vidalia onions for this recipe. Sweeter onions will be a bit higher in carbs.

Use 1/3 tsp. (or more to taste) ground rosemary if you don’t have fresh rosemary.

Use 1 tsp. (or more to taste) ground thyme if you don’t have fresh thyme.

I used a cheese blend that had Mozzarella, Provolone, Parmesan, and Romano. Any similar blend of cheeses will be fine.

This recipe was inspired by Onion Gratin at Closet Cooking and many creamed onion casseroles from my childhood.

Nutrition Information

Yield

8

Serving Size

1
Amount Per Serving Calories 322Total Fat 29gSaturated Fat 11gUnsaturated Fat 18gCholesterol 42mgSodium 280mgCarbohydrates 9gFiber 2gSugar 4gProtein 9g

Nutrition information is automatically calculated by the Recipe Plug-In I am using. I am not a nutritionist and cannot guarantee 100% accuracy, since many variables affect those calculations.

Did you make this recipe?

Did you make this recipe? Please leave a star rating (under the PRINT button in the recipe) or share a photo of your results on Instagram! THANKS!

Low-Carb Diet / Low-Glycemic Diet / South Beach Diet Suggestions:
Onions are high in carbs than some vegetables, but I bet this is lower in carbs than you might think and it should work for low-carb diets. The gratin topping here is better for the original South Beach Diet than the creamed onions of my childhood. But South Beach would recommend reduced fat mayo and cheese and this should probably be a “once in a while treat” for South Beach Dieters. 

Find More Recipes Like This One:
Use Side Dishes to find more recipes like this one. Use the Diet Type Index to find recipes suitable for a specific eating plan. You might also like to follow Kalyn’s Kitchen on Pinterest, on Facebook, on Instagram, on TikTok, or on YouTube to see all the good recipes I’m sharing there.

Historical Notes for this Recipe:
This recipe for Onion Gratin was posted in 2009, and I’ve made it for many Thanksgiving dinners since then! It was last updated with more information and a slightly-improved recipe in 2026.