Heat the Instant Pot Using the Sauté setting. When hot, add the onion and stir to cook for about two minutes.
Add the white wine (or broth) and garlic. Continue stirring and cooking for another two minutes, then press the Off button.
Add the mushrooms, soy sauce, nutritional yeast, mustard, onion powder, garlic powder, sage, tarragon, and one cup of the broth to the Instant Pot. Stir to combine. The mixture will look dry, but the mushrooms will release a lot of liquid as the cook.
Replace the lid on the Instant Pot. Using the Manual setting, cook under pressure for 20 minutes, then allow the pressure to fall naturally.
Stir the cornstarch into the remaining 1 1/2 cups of broth.
Remove the Instant Pot lid and press the Sauté button. Add the white pepper and cornstarch/broth mixture.
Stir as the gravy comes to a boil and begins to thicken. Mix in the cashew cream if using. If you want the gravy thicker, sprinkle a bit of flour and continue stirring or add a bit more cornstarch dissolved into 1/4 cup cold water.
Adjust seasonings. Serve hot.