Preheat your oven to 350°F (175°C). Lightly grease your baking dish. This helps prevent sticking and ensures even cooking of the potatoes.

Wash, peel and slice potatoes into thin rounds. Try to keep slices even. This helps them cook at the same time and gives a smooth texture.

In a saucepan, melt butter over medium heat. Add flour and stir for one minute. Slowly add milk and cream while whisking to make a smooth sauce.

Add garlic, salt, pepper and paprika to the sauce. Stir well. Let the sauce cook until it thickens slightly and becomes creamy.

Place a layer of sliced potatoes in the baking dish. Add some onion slices. Pour a portion of sauce over them. Repeat layers until everything is used.

Sprinkle cheddar and Parmesan cheese on top. Make sure the top layer is well covered. This will create a golden, cheesy crust.

Cover the dish with foil and bake for 35 minutes. Then remove foil and bake for another 15 minutes until top is golden.

Insert a fork into the potatoes. If they are soft, the dish is ready. The sauce should be bubbly and thick.

Let the dish rest for 10 minutes before serving. This helps the sauce set. Serve warm and enjoy the creamy texture.