1. Whirl pear, onion, ginger, garlic, sake, mirin, and soy sauce in a blender until puréed. Pour into a deep bowl, add short ribs, and chill 3 to 4 hours. Drain marinade and let meat come to room temperature, about 30 minutes.
2. Meanwhile, heat grill to medium (350° to 450°) for a gas grill or hot (450° to 550°) for a charcoal grill, with a burner turned off or an area left clear for indirect heat. In a bowl, toss kimchi with oil and set aside. Grill meat, covered, over direct heat, turning once, until marks appear, 4 minutes total.
3. Transfer meat to indirect heat area and set a metal rack over direct heat, crosswise over grates. Cook steak over indirect heat until a thermometer reaches 125° for medium-rare, 6 to 8 minutes. Meanwhile, grill kimchi on rack, turning once, until lightly charred, about 8 minutes total. Set kimchi on a platter with steaks on top, and serve with rice if you like.
*Short ribs don't come already boned, so you can ask a butcher to do it, bone them yourself (3 3/4 lbs. yields 1 1/2 lbs. trimmed "steaks"), or substitute boneless beef country-style ribs, a cut from the chuck eye roll that's similar in flavor, though less marbled. Find kimchi in grocery stores' refrigerated foods section and at Asian markets.