1. Preheat oven to 400°F (200°C) and butter a 9×13-inch baking dish.

2. Peel and slice potatoes into 1/8-inch rounds using a mandoline or sharp knife. Keep slices in cold water to prevent browning.

3. In a saucepan, melt butter over medium heat. Add minced onion and cook until soft, about 3-4 minutes. Add garlic and cook 1 more minute.

4. Sprinkle flour over onion and garlic. Whisk constantly for 1 minute to form a roux.

5. Slowly whisk in warm milk, then add heavy cream. Cook and stir until sauce thickens, about 5-7 minutes.

6. Remove from heat and stir in salt, pepper, and nutmeg. If using cheese, stir in 1 cup and mix until melted.

7. Drain and pat potatoes dry. Layer one-third of potatoes in prepared dish. Pour one-third of sauce over top. Repeat twice for three total layers.

8. Sprinkle remaining 1/2 cup cheese over top layer if using.

9. Cover with foil and bake for 45 minutes.

10. Remove foil and bake uncovered for 20-30 more minutes until golden and bubbly.

11. Let rest 10-15 minutes before serving to allow sauce to set.