Spread the bases of the rolls with the caramelised onion, top with the prosciutto and mozzarella and sandwich with the tops. Whisk together the eggs, milk and pepper in a bowl. Heat the butter in a large non-stick frying pan over medium heat. Dip the rolls in the egg mixture and cook for 2–3 minutes on each side or until golden and the cheese is melted. Serves 2.