- Unsalted butter to grease the pan
- 3 pounds Yukon Gold potatoes peeled
- 3 tablespoons unsalted butter
- 3 tablespoons all-purpose flour
- 3½ cups whole milk
- Pinch ground nutmeg
- 1 teaspoon salt
- Freshly ground black pepper
- ½ onion very thinly sliced
- 2 tablespoons fresh thyme leaves
- 2½ cups grated Gruyère cheese
- ¼ cup grated Parmesan cheese
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Pre-heat the oven to 375°F. Grease a 9x13-inch baking pan or casserole dish with butter.
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Using a mandolin slicer or a sharp knife, evenly slice the potatoes into ⅛-inch thin slices. Pat the potato slices dry with paper towels or clean kitchen towel.
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Melt the butter in a medium saucepan over medium heat. Add the flour and stir into a paste to form a roux. Cook for a minute or two. Slowly pour in the milk and whisk until the sauce is smooth and has thickened. Stir in the nutmeg, salt and season with freshly ground black pepper to taste.
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Layer one third of the potatoes evenly in the bottom of the prepared pan. Top with one third of the sliced onions and one third of the fresh thyme leaves. Pour one third of the of the sauce over the potatoes and spread it out evenly over the vegetables with a spatula. Repeat two more times with layers of potatoes, onions, thyme and sauce. Sprinkle the Gruyère and Parmesan cheeses on top.
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Transfer the pan to the oven and bake at 375°F for 50 to 60 minutes until the cheese on top is golden brown and the potatoes are soft when pierced with a knife.
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Let the scalloped potatoes rest at least 15 minutes before serving. Serve warm.
Make ahead – If you assemble and bake these scalloped potatoes ahead of time, only bake for 45 minutes. Let it cool completely and then wrap well and store in the refrigerator. Remove it from the fridge 1 hour before re-heating. Cover with foil and reheat in a 350°F oven for about 30 to 35 minutes, removing the foil the last 10 minutes of the cooking time.
To prep ahead – boil the sliced potatoes for 2 minutes to stop the oxidation process, slice the onions and make the sauce. Store the ingredients separately. Assemble the day of and bake for 45 to 50 minutes.