Mushroom stockis a flavorful vegetarian base for Tortellini or French Lentil Soup.
Bring 4 cups water to a boil. Place the dried mushrooms in a medium heat-proof bowl; pour the boiling water over mushrooms. Let stand until softened, about 20 minutes. Strain through a fine sieve into another bowl, reserving liquid and mushrooms separately. Set aside.
In a medium stockpot, cook butter and oil over medium heat. Add onion, and cook until caramelized, about 20 minutes, stirring occasionally. Add the reserved mushrooms along with the carrots, parsnips, and celery; cook, stirring often, until vegetables are softened and fragrant, about 20 minutes.
Stir Swiss chard into vegetable mixture in pot. Add 3 1/2 quarts cold water, reserved mushroom liquid (being careful to leave behind any sediment), bay leaf, thyme, and parsley. Cover pot, and bring to a boil; reduce heat, and simmer, uncovered, about 1 hour.
Remove pot from heat, and strain mixture through a fine sieve or cheesecloth-lined colander into a large heatproof bowl or saucepan, pressing down on vegetables with the back of a wooden spoon to extract as much liquid as possible. Discard solids. Store stock in airtight containers in the refrigerator up to 3 days or in the freezer up to 6 months.