1. Drain any excess oil from pesto.
2. Cut mozzarella in half lengthwise; cut each half into 12 thin slices.
3. Cut ciabatta into 8 pieces, and then slice each piece in half horizontally.
4. 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.