Prepare ingredients by slicing the onion into thin slices and cutting the chicken into strips about an inch long. Also cook enough white rice to make 4 cups.
In a non-stick pan or well seasoned skillet, add onion, sugar, dashi, soy sauce, and mirin to a pan and bring to a simmer on medium heat, stirring occasionally to dissolve sugar and to make sure the onion doesn't stick to the bottom of the pan.
Once the sauce has come to a boil, reduce heat and continue to simmer for 5 minutes, stirring occasionally.
After 5 minutes, increase heat back to medium and add chicken to the mixture. Stirring occassionally, continue to cook for around 7 minutes.
While the chicken is cooking, beat the eggs in a separate bowl until fully combined.
After the chicken has cooked for 7 minutes, reduce heat to low and pour egg mixture evenly into the sauce and chicken mixture. Cover and cook undisturbed for 3 minutes.
While eggs are cooking, add one cup of rice into each bowl.
After 3 minutes, remove from heat and serve chicken and egg mixture over rice bowl, pouring any extra sauce into the rice.
While in Japan the mixture is added to the top of the rice and served as is, I like to mix my oyakodon into the rice prior to adding any garnish. This is primarily because I use a skillet that has a little larger base than the rice bowl I use and the oyakodon doesn't fit as well as it would using specialized oyakodon pots.
Garnish with togarashi and diced green onions. You can skip the togarashi if you don't like spicy food.
Serve and enjoy!