# What you'll need:
→ Vegetables
01 - 2 lbs Yukon Gold or Russet potatoes, peeled and thinly sliced
02 - 1 medium yellow onion, thinly sliced
→ Dairy
03 - 2 cups shredded sharp cheddar cheese
04 - 1 cup shredded mozzarella cheese
05 - 2 cups whole milk
06 - 2 tablespoons unsalted butter
07 - 2 tablespoons all-purpose flour
→ Seasonings
08 - 1 teaspoon salt
09 - 1/2 teaspoon black pepper
10 - 1/2 teaspoon garlic powder
11 - 1/4 teaspoon paprika
# Method:
01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or cooking spray.
02 - Melt 2 tablespoons butter in a medium saucepan over medium heat. Add 2 tablespoons flour and whisk continuously for 1 minute until a paste forms.
03 - Slowly pour 2 cups whole milk into the roux while whisking constantly to prevent lumps. Continue cooking until the mixture thickens, approximately 3 to 4 minutes.
04 - Remove from heat and stir in salt, pepper, garlic powder, and half of the shredded cheddar and mozzarella cheeses until completely melted and smooth.
05 - Distribute half the sliced potatoes evenly in the prepared baking dish. Layer with half the onion slices and half the cheese sauce. Repeat with remaining potatoes, onions, and sauce.
06 - Sprinkle the remaining shredded cheddar and mozzarella cheeses over the top layer and dust with paprika.
07 - Cover the baking dish tightly with aluminum foil and bake for 40 minutes in the preheated 375°F oven.
08 - Remove the foil and continue baking for an additional 25 to 30 minutes until potatoes are tender and the top achieves a golden brown color.
09 - Remove from oven and allow the casserole to rest for 10 minutes before serving.