Creamy Scalloped Potatoes

1. Heat the oven and prep the dish. Preheat to 375°F. Butter a 9x13-inch baking dish.
2. Slice the potatoes. Slice to about 1/8-inch. Keep slices as even as you can so they cook at the same speed.
3. Start the flavor base. In a medium saucepan over medium heat, melt the butter. Add the onion and cook 5 to 7 minutes until soft and lightly golden. Add garlic and cook 30 seconds, just until fragrant.
4. Make the sauce. Sprinkle in the flour and whisk constantly for 1 minute. Slowly whisk in the milk, then the cream. Add salt, pepper, thyme, and nutmeg. Simmer gently 3 to 5 minutes, whisking often, until the sauce thickens enough to coat the back of a spoon.
5. Add cheese (off heat). Turn off the heat, then stir in 1 1/2 cups cheddar and 3/4 cup Gruyere until fully melted and smooth. If it looks a little lumpy, keep whisking for a minute. It will come together. Save the rest for the top.
6. Layer (3 layers). Add one third of the potatoes to the dish, overlapping slightly. Spoon over one third of the sauce. Repeat two more times, finishing with sauce.
7. Top and bake covered. Sprinkle remaining cheddar and Gruyere on top, then Parmesan. Cover tightly with foil so the potatoes steam and soften (and so the top does not overbrown too soon). Bake 55 minutes.
8. Check doneness, then brown the top. Carefully peel back the foil and test the center with a knife. You want little to no resistance. Re-cover and bake 10 more minutes if needed. Once the potatoes are tender, uncover and bake 20 to 25 minutes more, until deeply golden and bubbling around the edges. If you want more color, broil 1 to 2 minutes, watching closely.
9. Rest. Let stand 15 to 20 minutes before slicing. This is where the sauce goes from “hot lava” to perfectly creamy.

Quick pan note: A standard 9x13-inch dish is ideal. If you use a smaller or deeper dish, plan on a longer bake time and rely on the knife test, not the clock.