Heat about 2 tablespoons of cooking oil in a pot over medium high heat. Saute onion and tomatoes until tender; add chicken pieces and cookk until chicken juice runs clear ( there should be no more blood oozing from the chicken).
Add the miso, mix and cook until the miso is softened and mushed up then add 4-5 cups of water, season with fish sauce to taste then turn up heat/fire to high, cover and bring mixture to a boil then lower heat to allow soup to simmer until chicken is tender; about 20 minutes.
Add okra then put lid back on and cook for 5 minutes, then take off lid again and add mustard leaves. Cover and cook just until mustard leaves are wilted.
Taste and adjust seasoning. Cover the pot, cook for a minute more then turn off heat. Serve hot with rice, lots of rice! Enjoy!!