Heat oil in pan to 380 degrees. Place 1 slice of proscuitto on a slice of bread. Cover with a slice of mozzarella. Season with peppercorns. Tear a sage leaf into 4 pieces and place one on each corner of the cheese. Cover with another slice of proscuitto, then mozzarella, and then bread.
Press down the sandwich firmly. Trim off crusts and cut sandwich into halves or quarters. Dip pieces into milk first, then flour, then eggs. Fry for 30 seconds on each side. Drain on paper towels and serve hot.