Put the inner pot in the Instant Pot base. Push the saute' button setting. Set the timer to thirty minutes. Heat the oil and brown the pork belly.
Add the red onion and tomato. Cook until wilted. Add the broth or water. Cancel the saute button setting. Cover the Instant Pot with the pressure cooker lid. Push the pressure cooker button setting. Set the pressure to LO and set the timer to five minutes. Note that I sliced the pork belly thinly and small, so I chose a five-minute cooking time. If you slice your meat thick, increase the cooking time.
Quickly release the pressure when done. Cancel the pressure cooker button setting and push the saute' button setting. Set the time to thirty minutes. Add the fish sauce and the sinigang mix powder. Season with more fish sauce or salt to balance the sourness. If you want it to be extra sour, add more sinigang mix. Mix to dissolve the powder. Let the soup simmer for two more minutes,
Turn off the saute' button setting and add the spinach