In a blender, puree the onion, scallions, garlic, soy sauce, orange juice, mirin, sesame oil, sugar and sesame seeds until smooth. Transfer the marinade to a large baking dish. Add the short ribs and turn to coat. Cover and refrigerate overnight. Bring the short ribs to room temperature before cooking.
Preheat the oven to 200. Light a grill or preheat a grill pan and brush with vegetable oil. Working in batches, grill the short ribs over high heat, turning once, until nicely browned and charred in spots, about 2 minutes per side. Transfer the ribs to a rimmed baking sheet and keep warm in the oven while you grill the remaining ribs. Serve the ribs with rice, macaroni salad and kimchi.