Oyako translates to "parent and child", and refers to the chicken and egg in the dish, while don is short for donburi - the rice and the bowl that the ingredients are served in. It's a filling, delicious one pan-dish that is made very quickly - probably in 20 minutes or less.
You can cook the ingredients in one skillet and then divide them between two rice bowls when it's ready to serve, but it looks nicer if you use two individual pans. This dish is so common in Japan that they have special oyakodon pans, large enough for one serving.
If you don't have dashi bags (which look like tea bags), use 200ml unsalted chicken stock for the liquid.
Put the dashi bag in a measuring cup and add 200ml (¾ cup and 1tbsp) of boiling water. Leave to steep for a few minutes. Lift the dashi bag out of the cup, then squeeze it to remove the liquid. Add the sugar to the hot liquid and stir to dissolve, then mix in the soy sauce and rice wine.
Mince the spring onions then set them aside for later. Halve the onion then thinly slice it. Remove and discard the bone from the chicken thigh, then cut the meat into one-bite pieces. Use chopsticks to gently whisk the eggs, but not too thoroughly.
Divide the dashi broth between two individual-serving skillets. Place the skillets over a medium flames and bring to the boil. Divide the onion between the skillets and let the pieces simmer until translucent, but not too soft, about two minutes.
Divide the chicken pieces between the skillets. Stir the ingredients and let them simmer until the chicken is cooked through, about five minutes.
While the chicken is cooking, heat the rice and put it into two individual-serving bowls. Keep the rice hot.
Pour the eggs over the chicken into the simmering broth, dividing it between the skillets. Stir the eggs briefly, then let them cook until very softly set. The mixture will be quite uneven - there will be some parts that are very liquidy and others that are more firm.
Slide the ingredients - including the liquid - out of the skillets over the rice. Scatter the spring onion over the top, then sprinkle with shichimi togarashi. Serve immediately, with Japanese pickles, if you like.