- Coat the steaks with 1/4 cup of the sauce in a glass dish. Let marinate, covered and refrigerated, while you are getting the grill hot (best not to let them marinate more than about 4 hours, because it will affect the color and texture of the meat).
- Light a gas grill to medium or prepare a charcoal fire, letting the coals burn until they are covered with a gray ash and are medium-hot. Lay the steaks on the hottest portion of the grill and let them sear for 4 minutes on one side, then turn and finish on the other (3 to 5 minutes more for medium-rare).
- While the steaks are cooking, brush or spray the onion slices with the remaining 1 tablespoon of oil. Arrange the slices on the grill around the steaks and cook, turning with a spatula, until lightly browned and crisp-tender, 6 to 8 minutes.
- As the steaks and onions are done, heat the remaining sauce to a simmer. Serve steaks as soon as they come off the grill with a generous spoonful of the heated sauce and a topping of the onions. Garnish with sprigs of cilantro.