Slice bitter melon in half lengthwise and remove seeds using a spoon. Thinly slice crosswise and place in a bowl; top with enough salted water to cover. Soak for 5 minutes; drain.
Combine dashi kombu, soy sauce, sake, mirin, and sugar in a small bowl until marinade is well mixed. Place beef short ribs in a microwave-safe bowl and pour in marinade; marinate for 30 minutes.
Stir bitter melon, onion, konnyaku, ginger, and garlic scape into beef mixture.
Cook beef in the microwave for 4 minutes.
Mix 2 teaspoons water and cornstarch together in a small bowl until dissolved; stir into beef mixture.
Cook beef in microwave for 1 minute more.
Spoon rice into bowls and top with beef and myoga.
