For potatoes:
Bring a pot of water to a boil.  Peel potatoes and cut into 4 pieces.  Put potatoes in boiling water and cook until you can easily pierce with a fork.

In the meantime heat olive oil in a heavy bottom pan.  Chop Onion and add to hot oil and brown slightly.  Add Ground beef to onions and cook through.  If you are using powdered beef stock, dissolve one tablespoon of stock in ½ cup of boiled water, if using a cube, dissolve one cube in ½ cub boiling water.  Add stock to beef and cook until liquid is completely reduced.  Season with pepper (there will be enough salt with stock).

Preheat oven to 450˚F

When the potatoes have cooked completely and are soft enough, mash with potato masher and add ½ cup milk, 1 tbsp butter and beaten egg.  Continue mashing with potato masher or hand held liquidiser until creamy and combined well.  Season with salt and pepper and set aside.

Slice mushrooms, in thick slices. In a small pan, melt 1 tblsp butter, add chopped garlic and brown slightly.  Add mushrooms and sauté for about 5 minutes.

In a deep roasting pan, line bottom of pan with ground beef.  Layer ground beef with mushrooms.  Distribute grated or sliced mozzarella evenly over mushrooms.  Top mozzarella layer with mashed potatoes.

Place dish in oven and cook until potato layer has browned.