Preheat oven to 450°. Place polenta slices on a baking sheet coated with cooking spray. Lightly brush with oil. Bake 5 minutes or until polenta is lightly browned. Turn polenta over and brush with oil. Bake 5 minutes longer. Sauté mushrooms in 2 tablespoons oil until soft. Add garlic, parsley, and tomatoes, and simmer 5 minutes, until thick. Season with salt and black pepper. Spoon over the polenta.