# 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
# How to Prepare:
01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or cooking spray.
02 - Melt 2 tablespoons unsalted butter in a medium saucepan over medium heat. Add 2 tablespoons all-purpose flour and whisk continuously for 1 minute to form a smooth roux.
03 - Slowly pour 2 cups whole milk into the roux while whisking constantly to prevent lumps. Continue cooking until the mixture thickens slightly, approximately 3 to 4 minutes.
04 - Remove from heat and stir in 1 teaspoon salt, 1/2 teaspoon black pepper, 1/2 teaspoon garlic powder, and half of the cheddar and mozzarella cheeses. Stir until completely melted and smooth.
05 - Arrange half of the sliced potatoes in the prepared baking dish. Layer with half of the onion slices and half of the cheese sauce. Repeat with remaining potatoes, onions, and cheese sauce.
06 - Sprinkle the remaining shredded cheeses and 1/4 teaspoon paprika over the top layer.
07 - Cover the baking dish 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, or until the potatoes are tender and the top is golden brown.
09 - Allow the casserole to rest for 10 minutes before serving to allow flavors to settle and facilitate clean portions.