Instructions

1. Heat the oven to 400ºF. Arrange a rack in the middle of the oven and heat the oven to 400ºF. Coat a 9x13-inch baking dish with 1 teaspoon of the unsalted butter.
2. Grate the cheese. Grate 8 ounces Gruyère or sharp cheddar cheese on the large hole of a box grater and set aside.
3. Thinly slice the potatoes and onions, and mince the garlic. Use a mandoline or a sharp chef’s to thinly slice 3 pounds Yukon gold or red potatoes into thin slices (no need to peel), doing your best to keep the potatoes slices stacked, as this will make assembly faster. Halve and thinly slice 1 small yellow onion. Finely chop 2 garlic cloves.
4. Make the cheese sauce. Place the remaining 4 tablespoons butter and the garlic in a large saucepan over medium heat. Once the butter is melted, add 1/4 cup all-purpose flour, 1 teaspoon kosher salt, 1 teaspoon Dijon mustard, and 1/2 teaspoon black pepper. Whisk to combine and cook until fragrant, about 1 minute. Add 2 cups whole milk and whisk vigorously until well-combined. Bring the mixture to a simmer, whisking occasionally. Remove from the heat, add the cheese, and whisk until mostly melted and smooth.
5. Layer the au gratin ingredients in the baking dish. Sprinkle the baking dish with about 2/3 of the sliced onions, arranging them in a single layer. Pour about 1 cup of the cheese sauce over the onions. Arrange the potatoes over the onions and cheese sauce — if you’ve kept them stacked, it is easy to fan them out into rows with the potatoes at an angle. Sprinkle the remaining onions over the potatoes, tucking them between rows if desired. Pour the remaining cheese sauce over the potatoes and spread into an even layer. Gently tap the baking dish against the counter to settle the sauce.
6. Bake for 60 to 75 minutes. Bake uncovered until the top and edges are browned and crisp, and a knife inserted in the middle can easily slide through the potatoes, 60 to 75 minutes.
7. Cool the gratin for 10 minutes before serving. Let the gratin cool for 10 minutes before scooping or slicing.