"Let's stop arguing if the egg or the chicken came first because for this dish, both are needed as main ingredients. Oyakodon is a Japanese rice bowl dish. It's a very common food at home. It's simple and easy to make."
Bring water and rice to a boil in a saucepan. Reduce heat to medium-low, cover, and simmer until rice is tender and water has been absorbed, 20 to 25 minutes.
Stir soy sauce, mirin, sake, sugar, and salt together in a microwave-safe bowl. Add chicken and stir to coat well; cover. Marinate in the refrigerator for 30 minutes.
Place bitter melon slices in a bowl of salted water. Let soak for 5 minutes. Drain.
Stir bitter melon and onion into the chicken mixture.
Cover bowl with plastic wrap and microwave on medium heat for 4 minutes. Uncover; pour in eggs. Cover bowl and cook until eggs are set, about 2 minutes. Remove from microwave; let stand for 2 minutes. Uncover and stir. An instant-read thermometer inserted into the chicken should read at least 165 degrees F (74 degrees C).
Divide rice between 2 serving bowls. Top with chicken and egg mixture. Garnish with myoga.