Form sandwiches: Bread, a slice of mozzarella, a slice of ham folded to fit inside the edges of bread, mozzarella and another slice of bread. Pinch edges of sandwich to seal them.
Add flour to a shallow bowl and season with garlic and onion.
Beat eggs in a dish with the Parm, lemon juice, salt and pepper.
Heat a large frying pan with a shallow layer of olive oil, 3-4 turn of the pan, over medium heat. Coat sandwiches in flour then in egg batter. Fry sandwiches 2 at a time until deeply golden, 6-7 minutes, turning occasionally. Add a bit more oil to the pan if necessary for the second batch of sammies.
Cut sandwiches corner to corner and serve.