Brown the beef in a large skillet over medium-high heat, breaking it up, until no longer pink, about 5 minutes. Drain all but 1 tablespoon of fat.

Reduce heat to medium. Add the onion and cook until softened, 3 to 4 minutes. Stir in the garlic and ginger, cook 1 minute.

Lower heat to medium-low. Add the gochujang, soy sauce, rice vinegar, brown sugar, sesame oil, and water. Stir until the sauce dissolves and coats the meat.

Bring to a gentle simmer, then reduce heat to low. Cook for 5 to 7 minutes, stirring occasionally, until the sauce is thick and glossy.

Toast the cut sides of the buns in a dry skillet or with a little butter until golden. Pile the filling onto the buns, top with scallions and sesame seeds, and serve.