Cheesy Potatoes Gratin

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  • Level: Easy
  • Total: 2 hr 5 min (includes resting time)
  • Active: 40 min
  • Yield: 8 to 10 servings
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8 tablespoons (1 stick) unsalted butter

1/2 yellow onion, finely diced

Kosher salt

2 cloves garlic, smashed and finely chopped

1/2 cup all-purpose flour

1 quart milk

1/4 cup Dijon mustard

2 cups grated Gruyere

1 cup grated Parmigiano

3 pounds Yukon gold potatoes, unpeeled, cut into 1/4-inch slices


  1. Preheat the oven to 375 degrees F.
  2. Melt the butter in a large saucepan over medium heat. Add the onions, then season with salt and cook until the onions are soft and very aromatic, 5 to 7 minutes. Add the garlic and cook for another 2 to 3 minutes. Whisk in the flour and cook until the roux is the consistency of wet sand, 3 to 4 minutes. Whisk in the milk, bring to a boil and cook for 5 to 6 minutes. Turn off the heat and whisk in the mustard and cheeses. Season with salt to taste to make sure it is delicious!
  3. Place a quarter of the potatoes in the bottom of a 9-by-13-inch baking dish. Ladle a quarter of the cheese sauce evenly over the potatoes. Repeat this process 3 or 4 more times with the remaining potatoes and sauce, pushing down the potatoes between each layer and topping with the sauce. Place the baking dish on a sheet tray lined with foil. Bake, rotating the dish halfway through, until the potatoes are soft when poked with a fork, 50 to 60 minutes. Tent with foil if the top is getting too brown.
  4. Let sit for 15 to 20 minutes before serving.
  5. Hot cheesy potatoes!