Prep all ingredients. Rinse the rice under water. Submerge dried shiitake mushrooms in hot water for 3-4 minutes to rehydrate them (if using). Slice ginger, shallot and mince garlic.
Turn on your pressure cooker and choose Sauté mode. Once the pot is hot, add a little olive oil. Add shallot, ginger slices and garlic and sauté until fragrant. Add both brown and white rice and continue to sauté for 30 seconds to a minute.
Add chicken, 4 1/2 cups of water (or half water, half chicken stock), dried shiitake mushrooms (if using) and salt to the pressure cooker. Switch to pressure cooking mode and cook for 25 minutes.
Once pressure cooking period is over, let the pressure cooker release pressure naturally. This may take 15-20 minutes.
Carefully open the lid, remove the chicken pieces and shred the meat. Discard the bones and skin.
Switch the pressure cooker to Sauté mode, gently stir, add a teaspoon of fish sauce (more or less to taste). Adjust the consistency of the congee by adding more water or heat it longer with Sauté mode.
Add chicken meat back to the pot to warm it through. Transfer congee to servings bowls, top with plenty of freshly cracked black pepper, scallions, cilantro and other toppings you like.