1. Combine gochujang, oil, and vinegar in a large resealable plastic bag.
2. Add a generous pinch of salt and grate in half of onion on the large holes of a box grater.
3. Add steak and seal bag, pressing out air, then rub steak to coat; chill at least 30 minutes and up to 12 hours.
4.Meanwhile, thinly slice remaining half of onion and combine in a large bowl with remaining ½ cup vinegar and a pinch of salt. Let sit until onion is softened, at least 30 minutes and up to 2 hours; drain.
5. Prepare a grill for medium-high heat; oil grate. Remove steak from bag, letting excess marinade drip off; discard marinade. Grill, turning once or twice, until lightly charred on the outside and medium-rare in the thickest part, 4–6 minutes, depending on the thickness of the meat. Transfer to a cutting board and let rest 10 minutes before thinly slicing against the grain.
6. Season meat with more salt and serve with tortillas, pickled onion, and more gochujang.
7. Serve with pickled onions and flour tortillas warmed according to package directions.