Grate the potatoes onto a clean tea towel along with an onion. Bring the edges of the towel together and squeeze the water out of the mixture into a bowl.
Add the potato - onion mixture to a bowl and mix in an egg and seasoning.
Fry the mixture in a non stick frying pan for 2-3 minutes on one side. Bring a plate over the top and flip the pan over, then slide it back in to cook the other side. Take the pan off the heat.
Spread passata onto the top of the hash brown base, then place chunks of mozzarella, cooked sausage, bacon, mushroom and tomato. Bake the whole thing for 15 - 20 minutes or until the whole thing is cooked through.