Preheat oven to 375 degree F. Prepare Pizza Dough as directed using the 16x12x1-inch pan. Arrange cheese slices on top of dough in pan.
In a large skillet, cook onions, covered, in 2 tablespoons olive oil over medium-low heat for 13 to 15 minutes or until onions are tender, stirring occasionally. Uncover; cook and stir over medium-high heat for 5 to 8 minutes more or until onions are golden. Remove from pan and set aside.
In the same skillet, combine mushrooms, remaining 1 tablespoon olive oil, the garlic, and rosemary. Cook over medium heat until mushrooms are tender; drain well. Spoon mushroom mixture over cheese. Top with onions.
Bake in the preheated oven for 25 to 30 minutes or until crust bottom is slightly crisp and brown. Cool in pan on a wire rack for 5 minutes. Sprinkle with parsley. Cut the pizza into 4-inch squares and serve immediately. Makes 12 servings.