In a large kettle over medium-high heat, blanch peppers in boiling salted water for 3-5 minutes. Drain and rinse in cold water; set aside. In a skillet over medium heat, brown beef and onion; drain. Add remaining ingredients and mix well. Loosely stuff into peppers. Place in a 9-inch square baking pan. Bake, uncovered, at 350 for 30-35 minutes or until peppers are done and filling is hot.