Heat 2 Tbsp. vegetable oil in a large Dutch oven or other heavy pot over medium-high. Cook one 2″ piece ginger, scrubbed, thinly sliced, and 2 garlic cloves, smashed, stirring often, until golden, about 4 minutes. Push ginger and garlic to one side of pot and add short rib meat and reserved bones. Cook, turning occasionally, until meat and bones are lightly browned, 5–7 minutes. Stir in 1 medium onion, thinly sliced, and 1 bunch scallions, trimmed, halved crosswise, and cook until heated through, about 2 minutes.