In a large pot, bring the water to a boil over medium heat.
Add the onions and tomatoes. Let them cook for a couple of minutes until the tomatoes soften.
Stir in the tamarind soup mix and fish sauce to the pot. Let it simmer for about 5 minutes to develop the flavors.
Add the radishes and continue simmering for another 5 minutes until they are slightly tender.
Gently add the fish pieces into the pot and cook for about 8-10 minutes, or until the fish is fully cooked and tender.
Stir in the kangkong (or spinach leaves) and green chili peppers (if using). Cook for an additional 2 minutes until the leafy greens are wilted.
Season with salt and pepper according to taste.
Serve the Fish Sinigang hot with steamed rice.