In a large bowl, combine the ground beef, onion and rice. Add salt, pepper and garlic powder. Mix well.
Shape the meat mixture into 1 1/2-inch meatballs
Heat the oil in a large skillet over medium heat. Add the meatballs and cook until browned on all sides.
In a medium saucepan, combine the tomatoes, tomato sauce, chopped green bell pepper, and cream of mushroom soup. Bring to a boil; reduce heat and simmer for 5 minutes.
Pour tomato sauce over porcupine meatballs in oven-safe skillet and then bake at 350 F for 45 minutes to 1 hour or until meatballs are cooked through and sauce is thick.