Preheat oven to 450 degrees. Heat a saucepan over medium heat. Coat pan with cooking spray. Add onion; saute 2 minutes or until translucent. Stir in chicken, stock, marinara and barbecue sauce; cook 3 minutes or until thoroughly heated. Remove pan from heat. Spread about 1/2 cup chicken mixture over each crust; top evenly with mozzarella cheese. Place pizzas directly on oven rack; bake at 450 degrees for 9 minutes or until cheese melts. Top with green onions and pepper. Cut each pizza into 4 wedges.