Combine the dashi, soy sauce, mirin, and sugar in a bowl and mix until the sugar dissolves. Set aside.

Cut the chicken thighs into bite-size pieces and marinate with 1 tablespoon sake for 5 minutes. Slice the onion lengthwise.

Crack the eggs into a bowl. Using chopsticks, lift the egg whites 5-6 times to break them into smaller clumps without fully whisking. You want visible contrast between whites and yolks.

Spread the onion slices in a single layer in a frying pan. Pour in the seasoning mixture and bring to a simmer over medium heat. Add the chicken on top and cook uncovered for 5 minutes until cooked through, flipping halfway.

Increase heat to medium. Drizzle two-thirds of the egg mixture in a circular pattern over the chicken and onions. When the egg is barely set, add the remaining egg and the mitsuba. Cook on medium-low until the egg is just set but still slightly runny.

Serve steamed rice in donburi bowls and slide the chicken-egg mixture on top. Drizzle with pan sauce and serve with shichimi togarashi on the side.