Rinse the ribs in cold water. Using a sharp knife, score the meat of each short rib on both sides in a criss-cross pattern, making sure not to cut all the way through the meat; scoring the meat helps the marinade to penetrate.
To prepare the marinade, thinly slice the green onion, crush the garlic cloves into a bowl, and peel and grate a 1-inch piece of ginger. Cut the kiwi in half, place it on a cutting board cut side down, and slice top-to bottom around the fruit to remove the skin. Grate the kiwi with a zester or fine grater into the bowl. If using an asian pear, peel it and thinly grate it. The kiwi/asian pear will tenderize the meat.
Add soy sauce, brown sugar, mirin, water, sesame oil and toasted sesame seeds, and black pepper to the bowl. Mix to combine. Transfer the marinade to a large ziplock bag or wide container, add the ribs, and massage the marinade in.
Let the ribs marinate overnight or for a minimum of 4 hours. If using a kiwi in the marinade, don't let them sit any longer than 24 hours or the flesh of the meat will begin to break down.