Preheat the oven:
Preheat your oven to 375°F (190°C).
Grease a 9×13-inch baking dish with butter or nonstick spray.
Prepare the sauce:
In a large saucepan over medium heat, melt the butter.
Add the chopped onion and garlic and sauté until soft, about 3 to 4 minutes.
Add the flour and cook for 1 to 2 minutes, stirring constantly to form a roux.
Prepare the cream sauce:
Gradually add the milk and heavy cream, making sure there are no lumps.
Cook, stirring constantly, until the sauce thickens, about 5 to 7 minutes.
Remove from heat and stir in grated cheddar, Parmesan, salt, pepper and paprika until cheese is melted and sauce is smooth.
Layer the potatoes:
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