Place the chopped onions in the Instant Pot with about 1/4 cup veggie broth and cook on the sauté function until they begin to brown. Stir occasionally. Add in all of the rest of the ingredients (EXCEPT the soy milk), stir everything well, and set the iPot to 4 minutes with QUICK RELEASE. Stir in the 1 cup of unsweetened soy milk. Remove about 2 cups of the soup, trying to get ALL of the potatoes (if you can), some mushrooms, and some broth. Blend in a high-speed blender until smooth and creamy, then replace into the pot with the rest of the soup. Stir and serve warm!