Heat the pot and put in the cooking oil. sauté the onion until the layers separate. Add the pork belly and cook until the outer part turns light brown. Put in the fish sauce and mix it with the ingredients. Pour in the water and bring it to a boil. Add the taro and tomatoes, then simmer for 40 minutes or until pork is tender. Put in the sinigang mix and chili. Add the string beans (and other vegetables if there are any) and simmer for 5 to 8 minutes. Put in the spinach, turn off the heat, and cover the pot. Let the spinach cook using the remaining heat in the pot. Serve hot.