In a large nonstick skillet, heat the oil over medium heat. Add the beef and onion and cook, breaking up the beef with a wooden spoon, until browned and cooked through, 6 to 8 minutes. Once browned, drain off any excess fat.

Reduce the heat to low and add the processed cheese, tomatoes and green chiles, and chile powder. Cook, stirring constantly, until the cheese melts and the dip is smooth and combined, 2 to 3 minutes. Sprinkle with cilantro, if you like. Serve with tortilla chips.