Whisk the eggs in a container with a spout until they're about half mixed, but there are still some areas with pure egg white and egg yolk.
In a small frying pan, about the diameter of the bowl you plan to serve your Oyakodon in, add the dashi, sake, soy sauce, sugar, and onions, breaking up any clumps of onion as you add them.
Bring the stock and onions to a boil and continue to cook until the onions are starting to turn translucent.
Add the chicken and turn down the heat to maintain a gentle simmer. Cook, turning the chicken over regularly until the surface is opaque, and then cover with a lid and cook until the chicken is cooked through (another 4-5 minutes).
Remove the lid and turn up the heat to bring the mixture to a boil. Drizzle the egg over the chicken in a figure eight pattern to ensure it's evenly distributed.
Smack the bottom of the pan against the burner a few times to redistribute the eggs.
Cover the pan with a lid and turn off the heat and let the egg steam while you serve the rice.
Add one serving of rice to a large bowl while the egg steams. When the egg is cooked to your liking, slide the chicken and egg onto the rice. Garnish your Oyakodon with scallions or Mitsuba and shichimi togarashi to taste.