Place ground beef and onion in 8 x 8 x 2-inch (1 1/2qt.) glass baking dish. Stir to crumble. Microwave on HIGH 6 to 7 minutes or until beef is cooked, stirring occasionally. Drain off drippings. Add remaining ingredients except cheese. Stir to combine, cutting tomatoes into pieces with spoon. Spoon filling into peppers, dividing evenly. Cover stuffed pepper with plastic wrap. Poke several holes in plastic wrap to allow steam to escape.