French Onion Soup Potatoes
When I was first thinking about making these French Onion Soup Potatoes, I wanted a side dish that captured the same comforting flavors of the classic soup but felt a little heartier. Swapping the soup bowl for a casserole dish turned out to be the perfect move. Think of it like au gratin potatoes, layered with caramelized onions, melty Gruyère cheese, and fresh thyme.
French Onion Soup Potatoes are rich and comforting, and they’re incredible alongside a simple roasted chicken, a perfectly cooked steak, or even braised short ribs. They also make a beautiful side dish for Thanksgiving turkey as well as for a Christmas roast or glazed ham, and they’re always a crowd-pleaser!
If you love comforting sides like these au gratin potatoes, here are a few more favorites to try.
Why You’ll Love This Onion and Potato Gratin
- Easy to make- Layers of potatoes, onions, and cheese come together quickly, and it’s mostly hands-off once the potatoes are in the oven.
- Caramelized onions- The onions are cooked low and slow until they develop a rich, sweet flavor that melts perfectly into the potatoes.
- Cheesy and gooey- As the Gruyère melts with the cream and milk, it thickens into a rich, luxurious layer of cheesy goodness.
- Make-ahead friendly- You can prep this dish a day in advance and refrigerate it overnight. I’ll share more on this in the tip section below.
- Crowd-pleaser- These potatoes disappears fast at the table and everyone ends up going back for seconds.
Ingredient Notes and Substitutions
- Gold potatoes- Tender yet firm, these are perfect for layering and baking. You can also use Russet potatoes.
- Butter and olive oil- I use both to cook the onions; butter for the flavor and richness, and olive oil to help prevent burning while caramelizing.
- Onions- I use yellow onions which are sweet and caramelize beautifully.
- White wine- Enhances the flavor of the onions. Use a dry white wine, preferably one that you would drink.
- Garlic- Use fresh garlic cloves.
- Fresh thyme- Brings a herby brightness that flavors every layer.
- Heavy cream and whole milk- Creates a creamy base for the potatoes.
- Gruyère cheese- Classic in french onion soup, it melts beautifully and gives a nutty flavor to the potatoes. Swiss can be a substitute if needed.
- White cheddar- Adds a sharpness and extra gooeyness. Other melty cheeses are fine too.
- Beef broth- Adds a savory flavor to the potatoes that mimics french onion soup.
- Nutmeg- Just a pinch brings a little warmth to the potatoes.
Step By Step Instructions
STEP 1: Cook the onions. Heat the olive oil and 2 tablespoons of butter in a large skillet over medium-high heat. Add the onions and cook, stirring occasionally, until softened, about 10 minutes. Stir in the fresh thyme, garlic, and season with salt and pepper. Next, deglaze with white wine and cook for 5 minutes, then add the beef broth and let it simmer until most of the liquid has evaporated.
STEP 2: Prepare the potatoes. Preheat oven to 350℉. Next, grease a 9×13-inch casserole dish with butter or nonstick spray. Then, peel the potatoes and and slice them into 1/8-inch thick, using a mandolin if you have one.
STEP 3: Assemble the layers. To start, whisk together the heavy cream, whole milk, and nutmeg in a mixing bowl. Then, spread 1/3 of the sliced potatoes evenly across the bottom of the prepared dish. Season with salt and pepper. Pour over 1/3 of the cream mixture. Sprinkle with 1/3 of the caramelized onions and 1/3 of the Gruyère and white cheddar cheese blend.
STEP 4: Continue layering. Repeat for the second and third layer. After the final layer, hold back the cheese. You’ll add it after the first bake.
STEP 5: Bake. Cover the dish tightly with foil and bake at 350℉ for 1 hour and 15 minutes, or until the potatoes are fork-tender in the center. Then, remove the foil , top with remaining cheese, including the Parmesan. Return the potatoes to the oven and bake uncovered for 20 minutes, until the top is golden and bubbly.
STEP 6: Rest and serve. Let the potatoes rest for 5-10 minutes before serving.
Tips For Success
- Don’t rush the onions. Caramelizing takes time, but it’s where all the flavor comes from. Keep the heat medium and stir every so often to prevent burning.
- Use a mandolin for even slices. A mandolin makes it quick and easy to slice all the potatoes uniformly, so none of them end up overcooked while others are still raw.
- Cover for the first bake. Keeping it tightly covered traps in steam so the potatoes soften all the way through before browning.
- Season as you go. A little salt, pepper, and fresh thyme sprinkled over each layer makes a big difference in the final dish.
- Make it ahead. Assemble the casserole as directed and bake it for about half the suggested time, until the potatoes are partially cooked. Allow the dish to cool completely, then cover tightly and refrigerate overnight. When you’re ready to serve, take it out of the fridge and let it sit at room temperature for about an hour. Bake until the potatoes are tender and heated through, then remove the foil, top with the remaining cheese, and bake for a final 20 minutes, until golden and bubbly.
Frequently Asked Questions
Scalloped potatoes are thinly sliced potatoes baked in a creamy sauce, and typically don’t include cheese. Au gratin potatoes, on the other hand, always include cheese and are typically sliced thinner than scalloped potatoes.
No, those two cheeses give the classic flavor and gooey texture, but you can swap in other melty cheeses if you like. Good options include Fontina, Emmental, or even a mild Swiss. Just avoid very hard cheeses that don’t melt well.
Cover with foil for most of the bake and uncover only at the very end to add the cheese and get a golden top.
The best way is to test the center with a knife or skewer, it should slide in easily without resistance. The edges may look slightly firmer than the middle, but as long as the potatoes in the center are tender, they’re ready.
Let the casserole rest for 5-10 minutes or longer after baking. This helps the layers set so slices hold together.
Storing and Reheating
French Onion Soup Potatoes keep well in the refrigerator for up to 3-4 days. Cover the casserole tightly with foil or plastic wrap to prevent it from drying out.
Reheating: For best results, reheat in the oven. Preheat to 350℉ and cover with foil. Bake until heated through in the center, about 25-30 minutes, depending on your oven and the size of the dish.
Looking For Other Easy Thanksgiving Sides?
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French Onion Soup Potatoes
Ingredients
- 2 tbsp unsalted butter
- 1 tbsp olive oil
- 5 medium yellow onions, peeled and sliced thin
- 1 tbsp thyme leaves , roughly chopped
- 2 cloves fresh garlic, finely minced
- 1/4 cup dry white wine
- 1/3 cup beef broth
- 2 1/2 lbs gold potatoes, peeled and sliced thin on a mandolin
- 1 1/2 cups heavy cream
- 1/2 cup whole milk
- 1 cup Gruyere cheese, hand grated
- 1 cup white cheddar cheese, hand grated
- 1/4 cup Parmesan cheese, hand grated
- salt and pepper, to taste
Instructions
- Heat the olive oil and 2 tablespoons of butter in a large skillet over medium-high heat. Add the onions and cook, stirring occasionally, until softened, about 10 minutes. Stir in the fresh thyme, garlic, and season with salt and pepper. Next, deglaze with white wine and cook for 5 minutes, then add the beef broth and let it simmer until most of the liquid has evaporated.
- Preheat oven to 350℉. Next, grease a 9×13-inch casserole dish with butter or nonstick spray. Then, peel the potatoes and and slice them into 1/8-inch thick, using a mandolin if you have one.
- To assemble, whisk together the heavy cream, whole milk, and nutmeg in a mixing bowl. Then, spread 1/3 of the sliced potatoes evenly across the bottom of the prepared dish. Season with salt and pepper. Pour over 1/3 of the cream mixture. Sprinkle with 1/3 of the caramelized onions and 1/3 of the Gruyère and white cheddar cheese blend.
- Repeat for the second and third layer. After the final layer, hold back the cheese. You'll add it after the first bake.
- Cover the dish tightly with foil and bake at 350℉ for 1 hour and 15 minutes, or until the potatoes are fork-tender in the center. Then, remove the foil , top with remaining cheese, including the Parmesan. Return the potatoes to the oven and bake uncovered for 20 minutes, until the top is golden and bubbly.
- Let the potatoes rest for 5-10 minutes before slicing and serving.