Put dashi soup stock in a pan and cook on medium heat. Add soy sauce, mirin, and sugar in the soup and bring to a boil. Stop the heat. To cook 1 serving of katsudon, put one quarter of the soup in a small skillet. Add one quarter of onion slices in the soup and simmer for a few minutes on medium heat. Add 1 serving of tonkatsu pieces in the pan and simmer on low heat for a few minutes. Beat an egg in a bowl. Bring the soup to a boil on medium heat, then pour the egg over tonkatsu and onion. Turn the heat down to low and put on a lid. Turn off the heat. Serve 1 serving of steamed rice in a large rice bowl, then place the simmered tonkatsu on top of the rice. Repeat the process.