Place oats, green onions (white parts), garlic, ginger, salt, chicken bone broth and water in the Instant Pot and stir to combine. If you have a cotton nut-milk bag (like this one) or a piece of cheesecloth, place the chicken in that and tie it up tightly before adding it to the pot. If not, simply add the chicken pieces to the other ingredients and stir to combine.
Lock the lid and program the Instant Pot to PRESSURE COOK, HIGH for 20 minutes.
While the congee is cooking, wash and thinly slice the bok choy; set aside. Trim the mushroom stems and slice the mushrooms thinly; set aside.
Once the cooking time finishes, let the pressure reduce naturally for 10 minutes. Then release any remaining pressure by shifting the pressure valve to VENTING. (Cover the valve with a kitchen towel to catch the steam.) Once the pressure has been fully released, carefully open and remove the lid.
Using kitchen tongs, gently lift the chicken out of the pot and place on a plate. If using a cotton bag or cheesecloth, untie this, pull out the chicken pieces (careful, hot!) and set them on the plate to cool. Squeeze the broth out of the cotton bag into the pot before rinsing it.
While the chicken is cooling, briskly stir the congee with a wire whisk to break down the grains (oats will retain a little schape, whereas rice will almost completely dissolve into the broth.)
Select SAUTE, NORMAL to reheat the congee. Add the sliced mushrooms and cook for 5 minutes until soft. Add the sliced bok choy and cook another 2-3 minutes until soft. Stir occasionally to prevent the congee from sticking to the bottom.
While the vegetables are cooking, remove the chicken meat from the bones and shred coarsely. Once the vegetables are done, gently stir the meat back into the porridge.
Season to taste with fish sauce and salt. Stir gently so as not to break up the chicken meat.
Serve into individual soup bowls and garnish as desired. I usually load mine up with the reserved scallion greens (finely chopped), fresh cilantro, a pinch of red pepper flakes, a tablespoon of coarsely chopped peanuts or toasted sesame seeds, a teaspoon of crunchy umami topping and an additional drizzle of sesame oil.