Form the 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 the edges of the sandwiches to seal them.
Add flour to a shallow bowl and season with the garlic and onion.
Beat the 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 turns of the pan, over medium heat. Coat the sandwiches in flour, then in egg batter. Fry the sandwiches, two 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 the sandwiches corner to corner and serve.