– Melt the butter in a skillet over medium heat. Add the flour, followed by the milk. Continuously stir the mixture until the flour is dissolved. Season it with salt, ground black pepper and grated Parmesan cheese. Stir well.
– Add potato slices to the white sauce and stir to coat all slices evenly. Turn off the heat. Transfer the potato slices to a baking dish and pour the remaining white sauce over them. Top with mozzarella cheese.