Combine 3 Tbsp. gochujang, 3 Tbsp. oil, and 3 Tbsp. vinegar in a large resealable plastic bag. Throw in several generous pinches of salt and grate in half of onion on the large holes of a box grater. Add steak and seal bag, pressing out air, then rub steak to coat; chill at least 30 minutes and up to 12 hours.

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.

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.

Season meat with more salt and serve with tortillas, pickled onion, and more gochujang.