Preheat oven to 350. Prepare a 8" square dish with cooking spray; set aside. In a skillet, cook chicken and onions until chicken is no longer pink. Add mixed vegetables and sloppy joe sauce. Mix well. Bring to a boil. Reduce heat and simmer ten minutes, or until vegetables are cooked. Spoon mixture into prepared dish; set aside. Prepare mashed potatoes according to package directions using water, milk, and margarine. Spoon potato mixture around edges of dish. Bake for 25 minutes, or until bubbly and hot.