Rinse the short ribs under cold running water to remove any bone fragments. Pat them completely dry with paper towels.
In a blender, combine the soy sauce, brown sugar, mirin, sesame oil, Asian pear, kiwi, onion, garlic, ginger, and black pepper. Blend on high until the mixture is very smooth.
Place the ribs in a large, non-reactive dish or a zip-top bag. Pour the marinade over the ribs, ensuring each piece is well-coated. Cover the dish or seal the bag.
Refrigerate and marinate for at least 8 hours or overnight, but no more than 24 hours. The fruit enzymes will continue to tenderize the meat.
Preheat your grill to high heat, around 450°F to 500°F (232°C to 260°C). Clean and oil the grill grates well to prevent sticking.
Remove the ribs from the marinade, allowing excess to drip off. Place the ribs on the hot grill in a single layer.
Grill for 3 to 4 minutes per side, until nicely charred and cooked through. The thin cut means they will cook quickly.
Transfer the cooked ribs to a cutting board and let them rest for 3 to 5 minutes. Beef is safe to eat when the internal temperature reaches 145°F (63°C).
Using kitchen shears or a sharp knife, cut the ribs between the bones into individual pieces. Garnish with sliced green onions and toasted sesame seeds before serving.