Start by cooking short-grain or medium-grain rice in a rice cooker or your preferred method of preparing rice.
In a pot, combine water and dried kelp. Simmer on low for 10 minutes then remove the kelp from the pot.
Add the dried bonito flakes. Bring to a boil then reduce the heat to simmer for 30 seconds. Turn off the heat then set aside for 10 minutes.
Strain the dashi broth through a fine-mesh sieve. This yields about 1 cup of broth.
While this recipe calls for 2 servings, the oyakodon should be prepared individually for each serving. At this point, the measurements listed in the steps is for one serving or one bowl.
Heat a pan over medium heat. Add the sliced onions, 1/2 cup dashi broth, 1 1/2 tablespoon soy sauce, 1 tablespoon mirin, 1 tablespoon sake, 1/2 tablespoon sugar, and 1 portion of chicken thigh.
Simmer for 5 minutes then flip the chicken and cook until it is no longer pink, about 2 to 3 minutes.
While the chicken is cooking, whisk an egg in a small bowl. Drizzle the egg over the chicken then cover with a lid. Cook until the eggs are slightly set.
Serve over a bowl of rice and garnish with scallions and/or chili oil.
Repeat steps 6 through 9 for the second serving.