Wash and peel the potatoes carefully. Slice them into thin rounds. Thin slices cook faster and soak up the creamy sauce better. Set the sliced potatoes in a bowl of cold water so they stay fresh and do not turn brown.

Place a medium saucepan on medium heat. Add butter and let it melt slowly. Stir in chopped onion and cook until soft. Add the minced garlic and cook for one minute. The kitchen will start smelling wonderful.

Sprinkle flour into the saucepan with the onions. Stir well so the flour blends with the butter. Slowly pour in milk while whisking. Add heavy cream and keep stirring. Cook until the sauce becomes smooth and slightly thick.

Add salt, black pepper, and paprika to the sauce. Stir well so the flavors mix evenly. Taste the sauce and adjust seasoning if needed. A well-seasoned sauce makes the whole dish taste richer and more balanced.

Grease a baking dish lightly with butter. Place a layer of potato slices on the bottom. Pour some creamy sauce over them. Sprinkle a little cheddar cheese. Repeat layers until all potatoes and sauce are used.

Cover the baking dish with aluminum foil. Bake in a preheated oven at 375°F for about 40 minutes. This helps the potatoes soften and cook slowly inside the creamy sauce.

Remove the foil carefully. Sprinkle parmesan and remaining cheddar cheese on top. Bake uncovered for another 20 minutes. The top should become golden and bubbly.

Take the dish out of the oven and let it rest for 10 minutes. This helps the sauce thicken slightly. Sprinkle fresh parsley on top and serve warm.