In a stockpot, heat 2 teaspoons of the olive oil over medium-high heat. Add the mushrooms and saute until they begin to brown, 4-5 minutes. Push the mushrooms to the side of the pot and add the remaining 1 teaspoon oil, the onion, carrots, celery and garlic. Raise the heat to high and saute, stirring often, until the vegetables are deeply browned, about 10 minutes. (The browner the vegetables, the richer the flavor of the stock.) Add the water, parsley, thyme, bay leaf and salt. Bring to a boil, then reduce the heat to medium low and simmer, uncovered, for 25-30 minutes. Remove from the heat and let cool slightly.
Carefully strain the stock into a bowl through a colander or sieve lined with paper towels or cheesecloth (muslin). Use immediately, cover and refrigerate for up to 3 days, or freeze in airtight containers for up to 3 months. Makes about 6 cups.