Preheat oven to 400°F. Arrange vegetables (except greens, kombu, and dried mushrooms) on a large baking sheet. Roast them until many of their edges brown, about 30 minutes. If using frozen and partially thawed vegetables, this will take additional time.

Add roasted vegetables, kombu, dried mushrooms, and greens (if using) to the Instant Pot, pressing them down to keep them below the fill line. Fill the pot with water until level is even with the top of the vegetables (approximately 8 cups). Too much water means weak broth (and don’t salt the broth until you are ready to use it).

Lock lid, and set for 30 minutes on high pressure. When cooking time has completed, turn off cook setting and allow pressure to release naturally for 10 minutes. Allow to cool for a few minutes so that it’s not too hot to handle. Strain the broth from the pot into a wide, shallow container (see safety note).