Into a bowl, add the previously cooked potatoes and mash them with a fork, add the butter or margarine, the grated Parmesan cheese, mix very well, add the black pepper and parsley to taste, mix well again and set aside.
Into a small baking tin, add a drizzle of oil and evenly spread it all over the tin. Fully cover the bottom and the edges of the baking tin with the slices of bacon, then add half of the mashed potatoes we've set aside and evenly spread this mixture in the center of the tin, making it as flat as possible. Add half of the shredded mozzarella cheese and cover it with the rest of the mashed potatoes. Top it all with the rest of the mozzarella cheese and to finish it off, fold the bacon slices that were hanging out of the tray over the cheese.
Bake in a preheated oven at 200°C (392°F) for 30 to 40 minutes.
Unmold and garnish to taste.
Serve.