How to Make It
Combine vinegar, onion, salt, oregano, cumin, garlic, and 3 tablespoons of the oil in a large ziplock plastic freezer bag. Add sirloin; seal bag, and massage to ensure steak is well coated. Chill 4 to 12 hours.
Remove steak from marinade; discard marinade. Pat steak dry with a paper towel. Heat 1 tablespoon of the oil in a large cast-iron skillet. Add steak; cook to desired degree of doneness, 6 to 7 minutes per side for medium-rare. Remove steak from skillet; let stand 10 minutes.
Heat remaining 1 tablespoon oil in skillet. Add onions; cook, stirring occasionally, until lightly browned, 4 to 5 minutes. Add stock; cover and cook until onions are browned, tender, and liquid is nearly evaporated, about 3 minutes.
Slice steak against the grain; divide evenly among 4 plates. Top evenly with onion mixture, and sprinkle with cilantro.