Turn your Instant Pot to the sauté function and add the oil, onion, garlic, peppers, mushrooms, and walnuts. Sauté for about 5 minutes until veggies are softened.
Then add all remaining ingredients, close the lid, locking to the sealing position. Set your Instant Pot to high pressure for 20 minutes. (It will take about 10 minutes to build pressure before the pot begins counting down.) Once timer is complete, let it naturally release for 10 minutes, and then you may quick release the remaining pressure. When the pin drops, open the lid, serve and enjoy.