Drain any excess oil from pesto.
Cut mozzarella in half lengthwise; cut each half into 12 thin slices.
Cut ciabatta into 8 pieces, and then slice each piece in half horizontally.
Onto bottom section of each ciabatta, arrange 3 or 4 spinach leaves, 1 1/2 Tbsp. pesto, several shavings of Parmigiano-Reggiano, 3-4 slices mushroom, 1 slice proscuitto and 3 slices mozzarella. Add ciabatta tops and wrap tightly in foil or plastic wrap. Store in refrigerator until ready to serve.No ciabatta? No problem – use focaccia bread or even small crusty rolls. You can substitute any high quality cured or smoked ham for the proscuitto, and arugula is delicious in place of the spinach.