Prep the mackerel: Strain the canned mackerel through a fine mesh to separate the oil from the fish. Set both aside.
Sauté the aromatics: In a large pot, heat oil and sauté the onions until soft. Add ginger and garlic, cooking until fragrant and lightly browned.
Build flavor: Stir in the tomatoes and cook for 1 minute. Pour in the reserved mackerel oil and season with fish sauce and black pepper.
Make the broth: Add miso paste and 1.5 quarts of water. Let it boil until the miso is fully dissolved.
Cook firm vegetables: Add daikon radish and simmer for 5 minutes. Stir in the sinigang mix to sour and slightly thicken the broth.
Add soft vegetables: Toss in eggplant, okra, green chilies, and string beans. Let the soup come back to a boil.
Add the fish and more water: Pour in the remaining ½ quart of water, then gently place the mackerel pieces into the pot. Avoid stirring too much so the fish stays in large pieces.
Finish with greens: Add kangkong and cook for another 2–3 minutes until just wilted. Taste and adjust seasoning with fish sauce or pepper.