Au Gratin Potatoes

Steps

1. Heat the oven to 400°F and butter a 9x13-inch baking dish.
2. Slice the potatoes 1/8 inch thick, ideally on a mandoline. Halve the onion and slice it as thin as you can.
3. Melt the 4 tablespoons butter in a medium saucepan over medium heat. Add the garlic and stir 30 seconds, then whisk in the flour and cook 1 to 2 minutes, until bubbly but not browned.
4. Slowly whisk in the milk, then the salt, dry mustard and pepper. Cook, whisking often, 5 to 7 minutes, until the sauce bubbles and coats a spoon.
5. Toss the cheddar and Gruyere together. Take the sauce off the heat and stir in 2 cups of the cheese until smooth, saving the last cup for the top.
6. Layer one third of the potatoes and onion in the dish, overlapping the slices, and spoon one third of the sauce over them. Repeat twice, then press down gently so the top layer is coated.
7. Seal the dish with foil and bake 45 minutes. Uncover, sprinkle on the rest of the cheese and bake 30 to 40 minutes more.
8. The potatoes are ready when the top is deep golden, the sauce bubbles at the edges and a knife slides through the center with no resistance. Let rest 15 minutes before serving so the sauce can thicken.

Good to know

• Make ahead: bake it fully up to 2 days early, cool, cover and refrigerate. Reheat covered with foil at 350°F for 35 to 45 minutes, until hot in the center.
• Even slices cook at the same rate, which is why a mandoline helps here; always use its hand guard.
• Warm leftover servings in the microwave until hot all the way through.
• Swap: red potatoes work well with the skins left on, and Swiss or white cheddar can replace the Gruyere.