Turn on your pressure cooker to sauté mode. When the inner pot is hot add two tablespoons of olive oil with the diced onion (and celery, if using). Cook for about two minutes then add the grated or sliced ginger along with 2 tablespoons fish sauce. Stir with a wooden spoon to keep the ginger from burning. Cook for one to two minutes, scraping off any brown bits with your wooden spoon. The browned bits add flavor but you don't want to get a burn notice while the soup cooks so add a splash of stock if some of the stuck on bits prove difficult to scrape off.
Add the rice, chicken pieces and seven cups of stock. Stir ingredients together, secure and lock the lid on your pressure cooker. Cook on high pressure for 20 minutes then perform a quick release (see note about cooking time and pressure release).
Once the pressure has been released, remove the lid and set it aside. Pick out the chicken pieces with tongs and shred them with two forks and add them back to the soup. Adjust the seasoning according to your preference or depending on the type of stock you used (unsalted or low-sodium). If you use the cabbage stir it now at this point. Add the last cup of stock if you want a looser soup. Arroz caldo continues to thicken as it sits so I always end up adding more liquid (especially when heating up leftovers).
Ladle into bowls and top with the recommended garnishes.
Fish Sauce and Chicken Stock: I like Kitchen Basics unsalted chicken stock or Swanson Low-Sodium Chicken Stock. Start with two tablespoons fish sauce and add more only after cooking if you feel it needs it.
Chicken Stock Quantity: Use seven cups to cook the soup and add the eighth cup later. This will help reduce the time the food comes to pressure.
Cook Time: I cooked mine at 25 minutes high pressure with a quick release but 20 minutes is plenty of time to soften the rice to porridge texture. 15 minutes should also work if you're pressed for time but I would add three minutes of natural pressure release.
Optional Vegetables: These additions don't compromise the flavor or texture of traditional arroz caldo but they do add more nutrition.