Heat a large frying pan and add the mirin and sake. bring to a boil.
Add dashi, soy sauce and sugar, and bring back the the boil.
Lay the onion out in a single layer and place the chicken on top. Cover nad bring to a boil.
Skim off any scum or fat then reduce the heat and let simmer for 10 minutes.
Slowly drizzle the beaten egg around the pan and cook gently until the egg is cooked through. Shake the pan occasionally to prevent it sticking.
Sprinkle the spring onion then pour the omelette over the hot, cooked rice. Add as much broth as you like.