8 tbsp (1 stick) unsalted butter
½ yellow onion (finely diced)
2 cloves garlic (smashed and finely chopped)
½ cup all-purpose flour
1 quart milk
¼ cup dijon mustard
2 cups grated gruyere
1 cup grated parmigiano
3 pounds yukon gold potatoes (unpeeled, cut into 1/4-inch slices)
Preheat the oven to 375 degrees F.
Let sit for 15 to 20 minutes before serving.