In a large bowl, combine 3 tablespoons olive oil, the garlic, fish sauce, sugar, pepper and salt and stir until the sugar and salt are dissolved. Add the beef and toss to coat. Marinate for 20 minutes (or refrigerated, up to 3 hours).
In a large, heavy skillet, heat the remaining 3 tablespoons olive oil over high heat until smoking. Add the onions and stir-fry until slightly softened, about 4 minutes. Meanwhile, strain the beef, discarding the marinade. Add to the skillet and cook, tossing, for 6 to 7 minutes for medium-rare. Top with the cilantro.