Prepare the Gizzard, Liver, and Heart: Cook the chicken gizzard, liver, and heart separately until tender.
Cook the Chicken and Onion: In a pan, heat cooking oil and sauté sliced onion until translucent. Add chicken thigh and cook until browned.
Add Remaining Ingredients: Add the cooked gizzard, liver, and heart back into the pan. Season with salt, soy sauce, sake, mirin, and dashi stock.
Prepare the Eggs: Lightly beat the eggs, ensuring not to scramble them. Pour over the chicken mixture.
Finish Cooking: Cover the pan and let the eggs cook until just set, with streaks of white and yellow.
Serve: Serve the oyakodon over rice, garnished with spring onion and an optional egg yolk.