Preheat oven to 350º degrees.
In an 8x8 baking dish, place half of the mashed potatoes, spread and flatten them all around the bottom layer. I suggest you use a spatula.
Add all the beef mixture, spread and flatten very gently over the first layer of potatoes.
Add about 1/4 of the mozzarella cheese over the beef evenly.
Spread the rest of the potatoes over the mozzarella. This will be a little harder step as the mozzarella and the beef may stick to the spatula. Use a spoon to help you as well. My video can give you an idea of how I do it.
Top with remaining mozzarella.
Bake for 15 minutes or until the top is nice and golden brown. Remember that ovens vary so it could be a bit quicker or a bit longer. So keep your eye on it.