Begin by preparing the vegetables. Roughly chop the fresh mushrooms, dice the onion, mince the garlic, chop the celery, and chop the carrots.
Add all the ingredients to the Instant Pot: water, chopped mushrooms, diced onion, minced garlic, chopped celery, chopped carrots, bay leaf, thyme, salt, pepper, and soy sauce if using.
Secure the lid on the Instant Pot and set the valve to the sealing position.
Select the ‘Manual’ or ‘Pressure Cook’ option and set the cooking time to 30 minutes at high pressure.
Once the cooking time is complete, allow a natural pressure release for about 10 minutes. After that, carefully switch the valve to the venting position to release any remaining pressure.
Once all the pressure is released, open the lid. Use a strainer or fine mesh sieve to strain the broth into a bowl, discarding the solids.
Taste the broth and adjust seasoning with more salt or pepper if necessary.
Serve the mushroom broth warm or store it in the refrigerator for up to a week or freeze for longer storage.