Select the High Sauté setting on the Instant Pot (the timer will default to 30 minutes), add the oil, and heat for 1 minute. Add the onions and salt and stir with a wooden spoon, separating the onion layers and coating the slices with the oil. Cover the pot with the glass lid and let the onions sweat, without stirring, for 10 minutes.
Uncover the pot and, wearing a heat-resistant mitt, hold the rim of the inner pot in place while you stir the onions vigorously, using the spoon to nudge loose any browned bits from the bottom of the pot. Leave the pot uncovered and set a timer for 4 minutes. Stir the onions vigorously again, making sure to nudge loose any browned bits. Set the timer for 3 minutes, then stir the onions vigorously again and continue to stir them at 2-minute intervals until the 30-minute Sauté cooking program has ended. Wearing heat-resistant mitts, lift out the inner pot.
The onions can be used right away, stored in an airtight container in the refrigerator for up to 4 days, or frozen for up to 3 months.
Select the Sauté setting on the Instant Pot, add the oil, and heat for 1 minute. Add the onion, garlic, carrots, celery, and salt. Sauté for about 10 minutes, until the vegetables give up some of their liquid and begin to brown just a bit. Stir in the tomato paste and nutritional yeast, then add 1 cup of the water and use a wooden spoon to nudge loose any browned bits from the bottom of the pot. Add the peppercorns, bay leaves, parsley, and remaining 7 cups water, making sure not to fill the pot more than two-thirds full.
Secure the lid and set the Pressure Release to Sealing. Press the Cancelbutton to reset the cooking program. Then select the Soup/Broth setting and set the cooking time for 10 minutes at High pressure. (The pot will take about 20 minutes to come up to pressure before the cooking program begins.)
Place a wire-mesh strainer over a large stainless-steel bowl. For a clearer broth, line the strainer with a double layer of cheesecloth.
When the cooking program ends, let the pressure release naturally for 30 minutes, then move the Pressure Release to Venting to release any remaining steam. Open the pot and, wearing heat-resistant mitts, lift out the inner pot and pour the broth through the prepared strainer into the bowl. Discard the vegetables. Let the broth cool to room temperature. (To speed the cooling process, set the bowl in a larger bowl containing an ice bath.)
The broth can be used right away, stored in an airtight container in the refrigerator for up to 3 days, or frozen for up to 6 months.
Select the Sauté setting on the Instant Pot, add the oil and garlic, and heat for 2 minutes, until the garlic is bubbling. Add the mushrooms and salt and sautéfor 3 minutes, until the mushrooms are wilted. Stir in the broth, using a wooden spoon to nudge any browned bits from the bottom of the pot. Stir in the buckwheat, onions, and pepper, making sure all of the buckwheat is fully submerged in the broth.
Secure the lid and set the Pressure Release to Sealing. Press the Cancel button to reset the cooking program. Then select the Manual or Pressure Cook setting and set the cooking time for 15 minutes at High pressure. (The pot will take about 10 minutes to come up to pressure before the cooking program begins.)
When the cooking program ends, let the pressure release naturally for 10 minutes, then move the Pressure Release to Venting to release any remaining steam. Open the pot and use a fork to fluff the buckwheat. Spoon onto plates, garnish with parsley, and serve warm.