Heat the oil in a 2-quart saucepan over medium heat. Add the onion and cook for 5 minutes or until tender, stirring occasionally.
Stir the soup, milk, chile pepper and tomatoes in the saucepan. Reduce the heat to low. Cook and stir for 3 minutes or until the mixture is hot and bubbling.
Lightly oil the grill rack and heat the grill to medium. Grill the beef for 15 minutes for medium or to desired doneness, turning the beef over once during grilling. Spoon the soup mixture over the beef and sprinkle with the cilantro, if desired.