Mix the dashi, mirin, sake, soy sauce and sugar together until the sugar dissolves. Put the chicken thighs and onion in a small pan, and pour over half the dashi broth so the chicken is just covered.
Slowly bring the mixture to the boil, skimming off any fat or scum that floats to the surface, and cook for 8-10 mins until the chicken is cooked through and the liquid has reduced slightly. Taste the broth for seasoning, and adjust if needed. Crack the eggs into a bowl, beat well, and drizzle over the chicken in the pan. Cook over a medium heat until the egg has thickened slightly (it’s best served runny).
Warm the remaining dashi broth in a separate pan. Serve the oyakodon over hot rice, with the extra dashi broth poured over. Season with mitsuba and togarashi or sansho pepper.
