Heat about 2 tablespoons of cooking oil in a pot over medium high heat.
Saute onion and tomatoes until tender.
Add chicken pieces and cook 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!!