Cut onion into 1/2-inch thick pieces, carrot into thin half-rounds, and potato and chicken into bite-size pieces.
Make dashi using a dashi packet or Katsuobushi bonito flakes, and set aside.
Meanwhile, heat oil in a frying pan, then cook chicken pieces until browned. Add all the cut vegetables and stir-fry for a couple of minutes. Sprinkle curry powder and stir. Pour the dashi into the pan and cook for a few minutes until vegetables are tender. Add soy sauce and mirin. Add salt to taste, if desired.
Mix potato starch and water to make a slurry. Then add the slurry to the soup, stirring constantly to prevent lumps. Cook for a couple of minutes until the sauce thickens.
Serve steamed rice on a plate and pour the curry over the rice. Serve immediately.