French Onion Mashed Potato Bake – Easy Cheesy Comfort Food

This French Onion Mashed Potato Bake takes all the cozy, savory flavors of classic French onion soup and combines them with fluffy mashed potatoes. It’s the perfect side dish for holidays, potlucks, or any comforting dinner.

The onions are slowly caramelized to bring out their natural sweetness, then folded into a creamy potato mixture with plenty of cheese. A final broil creates a golden, crispy crust that everyone will love.

Not only is this dish delicious, but it’s also make-ahead friendly, so you can prepare it in advance and pop it in the oven when you’re ready.

About the Ingredients

3 large yellow onions (about 3 cups sliced)

Yellow onions caramelize beautifully; use sweet onions like Vidalia for a milder flavor if preferred.

3 tablespoons unsalted butter

Butter adds richness; you can substitute olive oil for a dairy-free option.

1 teaspoon fresh thyme leaves (or 1/2 teaspoon dried)

Thyme complements the onions; feel free to use rosemary for a different herb note.

3 pounds russet potatoes, peeled and quartered

Russets yield fluffy mashed potatoes; Yukon Golds also work well for a creamier texture.

4 ounces cream cheese, softened

Cream cheese adds creaminess; use full-fat for best results, or Greek yogurt for a tangy twist.

1/2 cup sour cream

Sour cream adds tang; you can substitute with plain yogurt or crème fraîche.

1/2 cup whole milk, warmed

Warm milk incorporates better; use any milk you have on hand, or even chicken broth for extra flavor.

1 1/2 cups shredded Gruyère cheese, divided

Gruyère melts wonderfully and has a nutty flavor; Swiss cheese is an excellent substitute.

1 cup shredded sharp cheddar cheese, divided

Sharp cheddar adds a bold taste; try Gouda for a smoky twist.

1 teaspoon salt, plus more to taste

Salt enhances all flavors; adjust according to your preference.

1/2 teaspoon black pepper

Freshly ground black pepper is best; add a pinch of cayenne for a little heat.

Tips & Tricks

Caramelize the onions low and slow

Don't rush this step. Cook them on medium-low heat for at least 20 minutes, stirring occasionally, until deeply golden and jammy.

Don't overmix the potatoes

Overmixing can make them gluey. Gently fold in the ingredients until just combined.

• Make sure your cream cheese and sour cream are at room temperature: This prevents lumps and ensures a smooth texture.

Step-by-Step Instructions

1. Caramelize onions

In a large skillet over medium-low heat, melt butter. Add sliced onions and thyme, and cook for 20-25 minutes, stirring occasionally, until onions are soft and deep golden brown. Season with a pinch of salt. Set aside.

2. Preheat & prep potatoes

Preheat oven to 375°F (190°C). Place peeled and quartered potatoes in a large pot and cover with cold salted water. Bring to a boil and cook for 15-20 minutes, until fork-tender.

3. Mash the potatoes

Drain potatoes and return to the pot. Add cream cheese, sour cream, and warm milk. Mash until smooth and creamy. Season with salt and pepper.

4. Combine onion mixture

Fold the caramelized onions and half of the Gruyère and cheddar cheeses into the mashed potatoes. Stir until evenly distributed.

5. Assemble the bake

Transfer the potato mixture to a buttered 9×13-inch baking dish and spread evenly. Sprinkle the remaining cheeses over the top.

6. Bake until bubbly

Bake in the preheated oven for 25 minutes, until the edges are bubbling and the top is lightly golden.

7. Broil for crispiness

Switch the oven to broil. Broil for 2-3 minutes, until the cheese is golden brown and crispy. Watch closely to prevent burning.

8. Rest and serve

Let the bake rest for 10 minutes before serving. Garnish with extra fresh thyme if desired.

Delicious Variations

Bacon Lover's

Add 1/2 cup cooked, crumbled bacon along with the onions for a smoky, salty crunch.

Veggie Boost

Stir in 1 cup of sautéed mushrooms or spinach with the onions for extra vegetables.

Herb Infusion

Replace thyme with 1 tablespoon of chopped fresh chives or parsley for a fresh, bright flavor.

How to Store French Onion Mashed Potato Bake

Frequently Asked Questions

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French Onion Mashed Potato Bake

French Onion Mashed Potato Bake combines caramelized onions and creamy potatoes with a crispy cheese top for the ultimate comfort side dish.

★★★★½ 4.8 (127 ratings)

15 minPrep time35 minCook time6Servings285 kcalCalories

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🧂 Ingredients

• 3 large yellow onions (about 3 cups sliced)
• 3 tablespoons unsalted butter
• 1 teaspoon fresh thyme leaves (or 1/2 teaspoon dried)
• 3 pounds russet potatoes, peeled and quartered
• 4 ounces cream cheese, softened
• 1/2 cup sour cream
• 1/2 cup whole milk, warmed
• 1 1/2 cups shredded Gruyère cheese, divided
• 1 cup shredded sharp cheddar cheese, divided
• 1 teaspoon salt, plus more to taste
• 1/2 teaspoon black pepper

📋 Instructions

1. 1Caramelize onions — In a large skillet over medium-low heat, melt butter. Add sliced onions and thyme, and cook for 20-25 minutes, stirring occasionally, until onions are soft and deep golden brown. Season with a pinch of salt. Set aside.
2. 2Preheat & prep potatoes — Preheat oven to 375°F (190°C). Place peeled and quartered potatoes in a large pot and cover with cold salted water. Bring to a boil and cook for 15-20 minutes, until fork-tender.
3. 3Mash the potatoes — Drain potatoes and return to the pot. Add cream cheese, sour cream, and warm milk. Mash until smooth and creamy. Season with salt and pepper.
4. 4Combine onion mixture — Fold the caramelized onions and half of the Gruyère and cheddar cheeses into the mashed potatoes. Stir until evenly distributed.
5. 5Assemble the bake — Transfer the potato mixture to a buttered 9×13-inch baking dish and spread evenly. Sprinkle the remaining cheeses over the top.
6. 6Bake until bubbly — Bake in the preheated oven for 25 minutes, until the edges are bubbling and the top is lightly golden.
7. 7Broil for crispiness — Switch the oven to broil. Broil for 2-3 minutes, until the cheese is golden brown and crispy. Watch closely to prevent burning.
8. 8Rest and serve — Let the bake rest for 10 minutes before serving. Garnish with extra fresh thyme if desired.

👨‍🍳 Chef's Note: Chef's note: For an extra indulgent twist, stir in a handful of crispy fried onions on top just before serving.

Servings: 6

Cuisine: American

Course: Side Dish

Nutrition (per serving): 285 kcal · 12g protein · 22g carbs · 9g fat