In a bowl, mix the chicken pieces with 1 tablespoon of soy sauce and 1 tablespoon of sake. Allow it to marinate for 30 minutes.

In a small bowl, combine the boiling water with the instant dashi powder. Whisk in the sugar, the remaining tablespoon of soy sauce, and the remaining tablespoon of sake.

Crack the eggs into a medium bowl. Using chopsticks or a fork, gently mix in ½ teaspoon of the dashi mixture into the egg whites without breaking the yolks. Puncture the yolks and stir gently to create large yellow ribbons in the whites. Set this mixture aside.

In an 8-inch skillet over high heat, bring the sliced onion and the remaining dashi mixture to a rapid simmer. Cover and cook until the onion starts to soften, about 2 minutes.

Add the marinated chicken to the skillet and cook uncovered, stirring occasionally, until the chicken is fully cooked, which should take about 6 minutes.

Pour in half of the egg mixture, swirling the pan so the egg spreads evenly while gently moving the chicken and onion to create an even layer. Cook undisturbed until the eggs begin to set, about 2 minutes.

Add the remaining egg mixture, cover the skillet, and cook until the second layer of egg is just starting to set, about 2 minutes.

Divide the cooked short-grain white rice into individual bowls. Top each bowl with half of the oyakodon, pouring any remaining liquid from the skillet over the top.

Garnish with the sliced scallion and serve with shichimi togarashi.