Directions:
Preheat the oven: Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish.
Prepare the cheesy scalloped potatoes: In a medium saucepan, melt butter over medium heat. Stir in flour and cook for 1 minute. Gradually whisk in the milk and cook until the mixture thickens. Remove from heat and stir in the cheddar and mozzarella cheese, garlic powder, salt, and pepper.
Assemble the potatoes: Arrange the sliced potatoes in the prepared baking dish. Pour the cheese sauce over the potatoes, making sure they are evenly coated. Sprinkle grated Parmesan on top.
Bake the potatoes: Cover the baking dish with aluminum foil and bake for 45 minutes. Remove the foil and bake for an additional 15 minutes, or until the top is golden and bubbly.