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:
Spread a thin layer of cheese sauce on the bottom of the prepared baking dish.
Arrange a layer of potato slices on top of the sauce, overlapping slightly.
Sprinkle with some of the crumbled bacon.
Repeat the layers, ending with a layer of cheese sauce on top.
Cook the dish:
Cover the baking dish with aluminum foil and bake for 45 minutes.
Remove the foil and cook for an additional 30 minutes, or until the potatoes are tender and the top is golden and bubbly.
Garnish and serve:
Let the dish cool for 10 minutes before serving.
Garnish with chopped fresh parsley if desired.
Tips and variations:
Cheese variations: Use a combination of cheeses such as Gruyere, Mozzarella or Swiss for different flavors.
Add vegetables: Stir in sautéed mushrooms, spinach or peppers for added texture and flavor.
Spice it up: Add a pinch of cayenne pepper or red pepper flakes to the sauce for a spicy kick.
