Heat oil in a pot over medium heat. Add onion, garlic, and ginger; cook and stir until onions are translucent, about 5 minutes. Add tomatoes; cook and stir until mushy, about 5 minutes. Stir in miso paste and fish sauce; simmer until flavors combine, about 2 minutes.
Pour water into the pot; bring to a boil. Add fish fillets and radish; simmer until radish softens slightly, about 2 minutes. Stir in chile peppers, mustard greens, and kalamansi juice. Simmer, uncovered, until greens are wilted, 1 to 2 minutes.
