French onion and potatoes gratin
Golden, cheesy, and rich with deeply caramelized onions, this indulgent take on potatoes au gratin makes the perfect side dish for any holiday or cozy dinner.
Ingredients
4 tbsp salted butter, plus more for greasing
2 tbsp olive oil
5 large onions, thinly sliced
1½ tsp kosher salt, divided
Black pepper, to taste
1 tsp sugar
1 tbsp fresh thyme, chopped
¼ cup dry sherry
½ cup low-sodium beef broth
2 tbsp all-purpose flour
1½ cups heavy cream
1½ cups whole milk
¼ tsp freshly grated nutmeg
6 oz Gruyère cheese, shredded
6 oz Swiss cheese, shredded
5 large russet potatoes, peeled and thinly sliced (¼-inch thick)
¼ cup grated Parmesan cheese
Chopped fresh chives, for garnish
Instructions
1. Caramelize the Onions
Preheat oven to 375°F (190°C) and butter a 9×13-inch baking dish.
In a large skillet, heat 2 tbsp butter and olive oil over medium-low heat. Add the onions, 1 tsp salt, pepper to taste, and sugar. Stir to coat, cover, and cook for 10 minutes.
Uncover, increase heat to medium, and cook for another 10 minutes, stirring often. Add thyme, sherry, and broth, and continue cooking until the onions are deeply caramelized and liquid has evaporated, about 15 minutes more.
2. Make the Cheese Sauce
In a Dutch oven or large saucepan, melt the remaining 2 tbsp butter. Whisk in the flour and cook for 1 minute. Gradually whisk in the cream and milk. Add the remaining ½ tsp salt and nutmeg. Bring to a gentle simmer and stir until thickened, about 2–3 minutes.
Remove from heat and stir in all but ½ cup each of the Gruyère and Swiss cheeses until melted. Fold in the sliced potatoes to coat.
3. Assemble & Bake
Layer half the cheesy potatoes into the buttered dish. Spread half the caramelized onions on top. Repeat with the remaining potatoes and onions.
Sprinkle the top with the reserved cheeses and Parmesan. Cover loosely with foil (tent it so it doesn’t touch the cheese) and bake for 45–55 minutes, until potatoes are tender.
Uncover and bake for an additional 20–25 minutes, until golden and bubbling.
Let the dish rest for 20 minutes before serving. Garnish with chopped chives.
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- Pro Tip
Thin and even potato slices are key. Use a mandoline for perfectly uniform slices, which helps them cook evenly and absorb the rich cheese sauce. This also gives you that ideal creamy, tender texture in every bite.
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