Preheat oven to 425 degrees F. Place bread shell on a baking sheet. In a small bowl combine hoisin sauce, vinegar, and, if desired, crushed red pepper. Spread hoisin sauce mixture over bread shell; sprinkle with 1 cup of the cheese. Top with meatballs, sweet pepper, and green onions. Sprinkle with the remaining 1/2 cup cheese.
Bake for 12 to 15 minutes or until pizza is heated through and cheese is melted.