Pour this 1 creamy sauce over sliced russet potatoes into a glass casserole dish for a comfort side that feels like real comfort food

In a medium saucepan, melt the butter over medium heat. Add the minced garlic and cook for about 30 seconds, just until fragrant, stirring so it doesn’t brown.
Sprinkle the flour over the melted butter and garlic. Whisk continuously for 1 to 2 minutes to form a smooth paste (a roux). It should bubble gently but not darken; this step cooks out the raw flour taste.
Slowly pour in the heavy cream and whole milk while whisking constantly to avoid lumps. Keep whisking until the mixture is smooth and begins to thicken slightly, 3 to 5 minutes.
Stir in 2 cups of the shredded sharp cheddar cheese, a handful at a time, whisking after each addition until it melts into the sauce. Add the salt, black pepper, and smoked paprika if using. Taste carefully and adjust seasoning as needed; this is the one creamy sauce that will flavor all the potatoes.
With the glass casserole dish of sliced russet potatoes waiting, carefully pour this one hot, creamy cheddar-garlic sauce evenly over the potatoes, making sure it seeps down between the slices. Use a spoon or spatula to nudge the potatoes a bit so the sauce can work its way through.
Sprinkle the remaining 1/2 cup shredded cheddar cheese and the Parmesan (if using) evenly over the top of the sauced potatoes for a nice cheesy crust.
Cover the glass casserole dish tightly with aluminum foil and place it on the middle rack of the preheated oven. Bake for 35 to 40 minutes, until the potatoes are starting to turn tender when pierced with the tip of a knife.
Remove the foil and continue baking for another 20 to 25 minutes, or until the potatoes are completely tender and the top is bubbly and golden brown. If you like a deeper color on top, you can broil for 1 to 2 minutes at the end, watching closely.
Let the scalloped potatoes rest on the counter for at least 10 to 15 minutes before serving. This short rest helps the creamy sauce settle and thicken so each scoop holds together nicely. Serve warm, straight from the glass casserole dish.
Variations & Tips
For extra onion flavor, sauté 1 thinly sliced yellow onion in 1 tablespoon of butter until soft and lightly golden, then scatter it over the sliced potatoes before you pour the creamy sauce on top. If you like a little meat in your side dishes, crisp up 4 to 6 slices of bacon, crumble them, and tuck the pieces between the potato layers; the smoky bacon plays nicely with the cheddar and garlic. You can also swap in half smoked cheddar or a bit of Colby Jack for a milder, creamier taste. For a touch of heat, add a pinch of cayenne pepper to the sauce along with the black pepper. If you need to make this ahead, assemble the potatoes and sauce in the glass casserole dish, cover tightly, and refrigerate up to 24 hours; add an extra 10 to 15 minutes of baking time since it will be starting cold. Leftovers reheat well, covered, in a 325°F oven or gently in the microwave with a splash of cream to bring the sauce back to life. To lighten it slightly, you can replace the whole milk with 2% milk, but keep the heavy cream so the sauce still feels rich and comforting.