Combine the grated pear, onion, garlic, soy sauce, sugar, sesame oil, mirin, and pepper in a bowl or zip-top bag.
Add the short ribs and the chopped scallions, turning to coat every piece.
Marinate in the refrigerator for at least 4 hours, or up to 24 hours if using pear (limit kiwi-based marinade to 2 to 4 hours).
Remove the ribs from the fridge 15 minutes before cooking so they aren't ice-cold going onto the heat.
Grill version: cook over high heat on a charcoal or gas grill for 60 to 90 seconds per side, watching closely, since the sugar in the marinade burns fast on this thin a cut.
Oven version: broil on a rack set close to the top element for 3 to 4 minutes per side, door slightly ajar, until the edges char and the center is just cooked through.
Rest 2 minutes, scatter with sesame seeds and extra scallion, and serve with rice and lettuce.