Preheat the oven to 425°F. Prepare the ingredients for Vegetable Stock, cutting the vegetables into 2-inch chunks and leaving any fresh mushrooms whole. Place in a lightly oiled roasting pan or on a rimmed baking sheet and roast, stirring occasionally, until everything is well browned, about 45 minutes. Transfer the vegetables to a large saucepan or pot and add 8 cups cold water along with the parsley, thyme, bay leaf, peppercorns, and any dried mushrooms. Bring to a boil, partially cover, and simmer for 30 minutes. Strain well, cool uncovered, then refrigerate covered for up to 4 days or freeze for up to 6 months.