Heat oil in a nonstick skillet coated with cooking spray over medium heat. Add onion; sauté 20 minutes or until golden. Add mushrooms and rosemary; sauté 10 minutes. Sprinkle with salt and pepper; stir well.
Preheat oven to 450°.
Place pizza crust on a large ungreased baking sheet. Lightly coat crust with cooking spray. Spread onion mixture evenly over crust; sprinkle with cheese.
Bake at 450° for 15 minutes or until cheese is bubbly. Cut into 8 slices.