In a mixing bowl, combine the ground beef, egg, cornstarch, salt, diced onion and pepper. Form into 1 inch meatballs; about 20.
In a large skillet over medium heat, brown the meatballs; drain fat and set aside.
Heat the oil in a large saucepan over low heat. Pour in the pineapple juice and simmer for a few minutes.
In a small bowl, combine the 3 tablespoons of cornstarch, soy sauce, vinegar and water. Stir until smooth and pour into the pineapple juice. Add the sugar and simmer until thickened, stirring constantly.
Place the meatballs, pineapple pieces, green pepper, carrot and onion into the sauce mixture. Heat thoroughly.
