Step 1: Prepare the potatoes
- Preheat your oven to 375°F (190°C). Grease a 9×13-inch (23×33 cm) baking dish with butter or nonstick spray.
- Peel and thinly slice the potatoes to about 1/8-inch thickness. Use a mandoline for even slices if available.
Step 2: Make the garlic cheese sauce
- In a medium saucepan, melt the butter over medium heat. Add the minced garlic and sauté for 1 minute until fragrant.
- Sprinkle in the flour and whisk continuously for 1-2 minutes to create a roux.
- Gradually pour in the milk, whisking to avoid lumps. Bring the mixture to a simmer and cook until slightly thickened, about 2-3 minutes.
- Remove from heat and stir in the cheddar cheese, Parmesan, salt, pepper, and paprika (if using). Mix until the cheese is melted and the sauce is smooth.
Step 3: Layer the potatoes and sauce
- Arrange half of the sliced potatoes in an even layer in the prepared baking dish. Pour half of the cheese sauce over the potatoes, spreading it evenly.
- Repeat with the remaining potatoes and cheese sauce, ensuring all slices are well-coated.
Step 4: Bake the potatoes
- Cover the dish with aluminum foil and bake for 40 minutes.
- Remove the foil and bake for an additional 20-25 minutes, or until the potatoes are tender and the top is golden and bubbly.
Step 5: Garnish and serve
- Let the scalloped potatoes rest for 5 minutes before serving. Garnish with chopped parsley, if desired.