The Béchamel: Melt butter in a skillet over medium heat. Whisk in the flour and cook for about 6 minutes until it turns golden. Gradually whisk in the milk (one cup at a time) until smooth. Bring to a boil, then reduce heat and simmer for 10 minutes, stirring constantly. Season with salt, pepper, and nutmeg. Set aside.
Prep Oven: Preheat your oven to 400°F (200°C).
Mix Meatballs: In a large bowl, combine the ground beef, diced onion, parsley, paprika, salt, and pepper. Shape into uniform meatballs.
Assemble the Dish: Line the bottom and sides of a round casserole dish with the parboiled potato slices. Arrange the meatballs on top, then place additional potato slices between each meatball to create a decorative pattern.
Sauce & Cheese: Pour the warm Béchamel sauce evenly over the meatballs and potatoes. Sprinkle the shredded mozzarella over the top.
Bake: Bake for 15–20 minutes (since potatoes are already parboiled) until the meatballs are cooked through and the cheese is golden and bubbling.
Serve: Let it rest for 5 minutes before slicing to allow the sauce to set.