Place mince, chopped onion, egg and bacon in a bowl and combine well. Make 8 balls, then flatten 4 into a burger patty (I use a potato masher to do this!).
On the flattened patties, cover with mozzarella to taste (I find one ball of mozzarella between 4 does the job). Then follow previous step with flattening patties and place on top of the other and create a burger shape sealing all edges.
Place in oven for about 45 minutes (depending on size they can take anywhere from 30-60 minutes).