Set a large pot over medium-heat heat, then add oil to coat the bottom. Sauté the garlic, onions, and tomatoes, stirring constantly until slightly caramelized.
Add the sinigang mix, then pour in about 6 cups of water, using less than the packet suggests if you prefer a stronger broth flavor. Cover and bring it to a boil.
Add the radish, then cover and simmer on medium heat for about 10 minutes.
Before adding the fish, lightly season it with salt to bring out its natural flavors. Then add the fish, eggplant, ampalaya, and chili pepper. Cook for another 5 minutes until the vegetables are tender and the fish is cooked through.
Season with fish sauce or salt, if necessary, then add the kangkong. Cover and turn off the heat. Let the kangkong wilt in the hot broth for about 2 minutes before serving.
Notes
Bangus Belly: I've use the boneless belly portion of the milkfish, known for its delicate flavor and rich, fatty texture. You may also use other parts of the bangus (be careful of the small, fine bones) or use salmon or pompano as substitutes.
Sinigang Mix: This is a convenient seasoning packet with tamarind and other spices to achieve Sinigang's signature tangy flavor. You can substitute with natural tamarind pulp or paste, or a mix of lemon or calamansi with tomatoes.
Vegetables: I've used radish, eggplant, ampalaya (bitter melon), and kangkong (water spinach). You can also include other vegetables such as sitaw (yardlong beans), okra, taro (gabi), spinach, and ginger or lemongrass for a citrusy aroma.