Evenly slice russet potatoes.
Dip in water for 5 minutes then rinse and drain water.
In the meanwhile, add butter in a heated pan.
Sauce Preparation: Start with making roux
Melt the butter and add all purpose flour and cook for few minutes until the flour becomes aromatic and slightly starts changing color to brown.
Now pour milk and stir well.
Season with salt, pepper, and paprika.
Stir and mix well.
Now add cheese and stir to make a thick and smooth sauce.
Set the Baking dish: Layer with potato slices then evenly pour the sauce. Sprinkle cheese.
Repeat the layering process 2 times.
Wrap with the baking dish with Aluminum foil.
Bake in the oven for 1 hour at 220°C or 428°F.
Now take it out and carefully remove the foil.
Topped with mozzarella cheese.
Broil for 5 minutes to give symbolic gratin color.
Take it out, serve hot and enjoy!