Remove casing from sausage. Cook in a medium nonstick skillet over medium-high heat until browned; stir to crumble. Drain; pat dry with paper towels. Set aside.
Prepare grill. Place 1 crust on grill rack coated with cooking spray; grill 3 minutes or until puffy and golden. Turn crust, grill-mark side up, and spread with half of marinara sauce. Top with half of sausage, mozzarella, mushrooms, onion, and Parmesan. Cover and grill 4 to 5 minutes or until cheese melts and crust is lightly browned. Repeat with remaining crust and toppings.