3pounds yukon gold potatoespeeled and sliced into 1/4" slices
3tablespoons + 2 teaspoons unsalted butter
1teaspoon kosher salt
1/2teaspoon white pepper
3cupswhole milkwarmed (I usually heat it for 2-3 minutes in the microwave)
Preheat the oven to 375°
In a lage saucepan, melt 3 tablespoons of the butter over medium heat. Sprinkle in the flour, salt, white pepper and nutmeg and whisk together until thick and pasty. Continue to stir the flour mixture for one minute until bubbly and fragrant.
Add the warm milk a little at a time, stirring constantly and scraping the bottom and sides of the pan to prevent the flour mixture from sticking and burning. Continue stirring. Bring the sauce to a low boil over medium heat and boil for one minute, stirring constantly, until sauce thickens. Remove from heat.
Spray a 2 quart casserole dish with 3" sides with vegetable spray. Add 1/3 cup of sauce to the bottom of the casserole dish and spread it around evenly.
Layer 1/4 of the potato slices into the bottom of the dish. Sprinkle the potatoes with 1/3 of the chopped onions. Pour about 1/2 cup of sauce over the potatoes and onions and continue to layer this way: potatoes, onions sauce... you should have about 4 or 5 layers of potatoes. Finish the layering with the sauce. (Note: if the sauce at the end is very thick -- or you're running low on sauce, add about 1/4 - 1/2 cup of milk and whisk it into the remaining sauce to loosen it.)
Dot the top of the casserole with 2 teaspoons of butter, cut into small cubes.
Cover the casserole with a lid (or if there's no lid, use tin foil). Bake for 45 minutes. Remove the lid (or foil) and continue baking for an additional 30 minutes.
Remove from oven and let cool for 15 minutes before serving.