In a large skillet over medium heat cook bacon until crisp, about 5 minutes. Transfer to a paper towel; crumble; set aside. Pour off all but 4 Tbsp. bacon fat. Add mushroom, onions, and garlic; cook, stirring occasionally, until most of the liquid evaporates, 7 to 8 minutes. Stir in crumbled bacon, oregano, salt, and black pepper (yields 3½ cups). Preheat oven to 450°F. Place bread shell on a pizza pan or baking tray. Sprinkle cheese over shell. Spoon 1-3/4 cups drained mushroom bacon mixture over cheese. Top with tomatoes. Bake until hot and cheese is melted, about 10 minutes.