Heat a large skillet over medium-high heat. Cook and stir beef and onion in the hot skillet until browned and crumbly, 5 to 7 minutes; drain and discard grease. Add water and taco seasoning; cook and stir until water is evaporated, about 5 minutes.
Mix enchilada sauce, cream of mushroom soup, and Cheddar cheese together in a slow cooker; turn on High. Add ground beef mixture to enchilada sauce mixture. Cook, stirring occasionally, until sauce begins to bubble, 15 to 20 minutes. Decrease heat to Low or 'Keep Warm' setting to serve.
