Preheat your oven to 190°C (375°F). Grease a large baking dish with butter or non-stick spray. This helps the potatoes cook evenly and prevents sticking.

Peel and slice the potatoes thinly, about ⅛ inch thick. Try to keep slices even for uniform cooking. Place them in a large bowl of cold water to prevent browning.

In a medium saucepan, melt butter over medium heat. Add flour and whisk for one minute. Slowly pour in milk while whisking. Cook until thick and smooth.

Add salt, pepper and garlic powder to the sauce. Stir in ½ cup cheddar cheese until melted. Remove from heat and set aside. The sauce should be creamy and smooth.

Drain the potato slices and pat dry. Arrange half of the potatoes in the baking dish. Pour half of the cheese sauce over them. Repeat with remaining potatoes and sauce.

Sprinkle remaining cheddar cheese and Parmesan on top. Add paprika for a golden, slightly spicy topping if desired. This creates a crispy, attractive crust.

Cover the dish with aluminum foil. Bake in the preheated oven for 45 minutes. This allows the potatoes to cook through and the sauce to become creamy.

Remove the foil and bake for another 15 minutes. This browns the top and adds a crunchy texture. Check with a fork to make sure potatoes are tender.

Let the scalloped potatoes cool for five minutes before serving. This helps the layers set slightly. Serve warm as a comforting side dish.