Prepare the Bouquet Garni: Place a 4-inch square of cotton cheesecloth on a work surface. Stack the bay leaf, herbs, and peppercorns (if using) on top, gather the cheesecloth to enclose them, and tie a length of unwaxed cotton twine around the bundle to keep everything together, knotting the twine securely.

Prepare the Stock: In a large bowl, combine the onion, carrots, pars­nip, and olive oil and toss to coat the vegetables. Transfer to the prepared baking sheet and roast until just tender, about 30 minutes, stirring every 10 minutes to ensure even cooking.

Transfer the roasted vegetables to a large pot and add the garlic, mushrooms, salt, and 10 cups water. Bring to a boil over high heat. Decrease the heat to medium-low, add the bouquet garni, and simmer, partially covered, for about 1 hour. Strain through a fine-mesh sieve, pressing down on the solids to extract as much liquid as possible.