Heat the cooking oil in a pot.
Add the chicken and cook, stirring occasionally, until no longer pink. Add about a tablespoonful of fish sauce, stir, continue cooking until the mixture is almost dry.
Add the sliced onion and grated ginger. Cook, stirring often, until fragrant, about a minute.
Add the chayote cubes. Pour in about six cups of water. Bring to the boil, lower the heat, cover and simmer for about 15 minutes or until the chayote cubes are tender.
Add the dashi. Stir. Turn off the heat.
Take about a quarter cup of the broth, pour into a bowl, add the miso paste and stir until smooth and pourable. Pour the mixture into the pot. Stir. Taste. Add more fish sauce, if needed.
Drizzle in a little sesame seed oil. Sprinkle with the sliced scallions. Serve.