Cover cashews in water and soak for eight hours or overnight. Discard liquid. (You may skip this step if you have a high-powered blender like a Vita-mix.)
Cover shitake mushrooms with an inch of boiling water. Allow to soak for thirty minutes. Drain mushroom liquid into a measuring cup. Add enough water to bring the total liquid to equal one cup.
Turn the Instant Pot on sauté function and allow to become hot. Add peppers, onions, and celery and sauté until translucent and soft.
When the onion mixture is soft, stir in mushrooms and potatoes. Add mushroom water.
Cover and set the pressure valve to seal. Set the Instant Pot to Manual, high, for 1 minute. When the timer has counted down to zero, turn the IP off and release the pressure. Take care to keep your face and hands away from the pressure valve to prevent burns.
While the Instant Pot is coming to pressure, make the cashew cream sauce. Transfer one cup of cashews and one cup of water to a Vitamix or some other high-powered blender. If you don't have a Vitamix, you can use a regular blender but you will have to soak the cashews for at least four hours, preferably eight, then discard liquid. Add miso and blend until smooth. Set aside.
Once all the pressure has released, and the valve has dropped down, it is now safe to open the lid. Turn the Instant Pot to the OFF position and remove the lid. Change setting to SAUTÉ and set the temperature to low.
Stir in cashew cream. Add another half cup of water to the blender and blend on high to capture all the remaining cashew cream. Add to Instant Pot.
Stir in thyme and black pepper. Salt to taste if using**. Simmer for five minutes or until the cashew cream has thickened. Add water if needed. Taste and adjust spices, adding more thyme if desired. Serve hot with crisp bread.