Prep the Potatoes

1. Preheat the Oven: Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with butter or cooking spray.
2. Slice the Potatoes: Peel the potatoes (if you prefer) and slice them thinly, about 1/8 inch thick. A mandolin slicer works great for even slices, but a sharp knife works too.
3. Minimize Browning: Place the sliced potatoes in a bowl of water to prevent browning while you prepare the rest of the ingredients.

Make the Sauce

1. Cook the Garlic: In a medium saucepan, melt the butter over medium heat. Add the minced garlic and sauté for 1-2 minutes until fragrant. Be careful not to burn the garlic.
2. Create the Roux: Sprinkle the flour over the garlic and butter, whisking constantly for about 1 minute to create a smooth paste.
3. Add Cream and Milk: Slowly whisk in the heavy cream and milk, making sure to keep stirring to prevent lumps.
4. Season the Sauce: Add salt, pepper, and paprika (if using), and bring the sauce to a simmer. Continue to cook for 3-5 minutes, until it thickens slightly.
5. Add Cheese: Stir in the cheddar and Parmesan cheeses, allowing them to melt into the sauce. Once smooth and creamy, remove from heat.

Assemble the Scalloped Potatoes

1. Layer the Potatoes: Begin layering the sliced potatoes in the prepared baking dish. Overlap them slightly to ensure each layer is well-covered.
2. Pour the Sauce: Pour about one-third of the cheese sauce over the first layer of potatoes, spreading it out evenly. Repeat the process until you have three layers of potatoes and sauce.
3. Top with Cheese: Finish with a generous sprinkle of cheddar and Parmesan cheese on top.

Bake and Finish

1. Cover and Bake: Cover the baking dish with aluminum foil and bake for 40 minutes.
2. Uncover and Bake Again: Remove the foil and bake for an additional 20 minutes, or until the potatoes are tender when pierced with a fork and the top is golden brown.
3. Cool and Serve: Let the dish cool for about 10 minutes before serving. This helps the sauce set and makes it easier to serve.