Place ground beef and onion in a large, deep skillet. Cook over medium-high heat, stirring occasionally, until the beef is browned and the onion is softened. Drain and set aside.

Combine cheese, cooked ground beef mixture, and diced tomatoes with green chile peppers in a medium saucepan over low heat. Cook, stirring occasionally, until the cheese is melted and the dip is hot throughout. Serve warm.