Heat the Instant Pot using the Saute Normal function. Add the olive oil and allow it to heat for 30 seconds.
Add the mushrooms and saute, stirring occasionally, for 5 minutes, or until the mushrooms start to turn golden brown.
Add the water and miso paste. Whisk until the miso paste has fully dissolved.
Add the tofu, nori, green onions, and fish sauce, if using.
PRESSURE COOK:
Attach the Instant Pot lid and heat it using the pressure cook on manual HIGH pressure setting for 8 minutes. Make sure the pressure is in the “sealing” position.
When the Instant Pot has finished cooking, release the pressure manually by switching the valve to the “venting” position. Be careful, the vent will steam and may splatter.
SERVE:
Open Instant Pot, ladle the soup into bowls, and garnish with sesame seeds (optional). Serve.