Melt butter in a skillet over medium heat. Add onion; cook and stir until translucent, 5 to 10 minutes. Add ground beef; cook and stir until browned and crumbly, 5 to 10 minutes.
Transfer beef mixture to a slow cooker. Stir in water, cabbage, tomato purée, condensed soup, rice, bouillon, salt, and pepper. Cover and cook on High for 8 hours.