Heat a nonstick skillet over medium-high heat. Add the ground beef and diced onion, breaking the beef apart with a spatula as it cooks.
Cook until the beef is no longer pink, about 6–8 minutes, then drain any excess grease using a colander.
Transfer the drained beef and onion mixture to a 4- to 6-quart slow cooker.
Pour the 15-ounce can of red enchilada sauce over the beef, then add the 16 ounces of cubed Velveeta on top.
Cover and cook on LOW for 1½ to 2 hours, stirring once halfway through, until the cheese is fully melted and the mixture is smooth.
Stir to combine, sprinkle with chopped cilantro if using, and serve warm with chips or desired dippers.