Adapted from Bon Appétit
Rich & flavorful with boosts from miso, mushrooms & kombu
Transfer vegetable mixture to a large pot. Add peppercorns and 4 quarts cold water. Bring to a boil, then reduce heat and simmer, uncovered and stirring occasionally, until broth is reduced by half, 60 to 75 minutes.
Let broth cool, then strain through a fine-mesh sieve into a large container, pressing on solids to extract as much liquid as possible; discard solids.
Do Ahead: Broth can be made 3 days ahead. Let cool; cover and chill, or freeze up to 3 months.