PREHEAT oven to 230°C.
STACK 2 sheets of phyllo pastry on a cutting board. Spray with olive-oil cooking spray. Top with 2 more sheets; spray again. Cut to make 2 (22cm by 17cm) rectangles. Repeat with 4 more sheets. Transfer to baking-paper-lined large baking trays and bake for 5 minutes or until crisp.
MEANWHILE, toss 4 cups baby spinach; 2 cups halved cherry tomatoes; 1 cup sliced mushrooms; ½ small red onion, very thinly sliced; 2 tbsp olive oil; and ½ tsp salt in a large bowl.
DIVIDE 1 cup grated mozzarella cheese and vegetable mixture among crusts. Bake for 3 to 4 minutes or until cheese melts. Top with shaved Parmesan cheese, if desired.