Heat the olive oil in a stockpot. Add the mushrooms, celery, onion, leek, tomato and garlic and cook over high heat, stirring, until the vegetables begin to wilt, about 5 minutes. Add the thyme, parsley, bay leaf, peppercorns, salt and 3 quarts of water and bring to a boil. Lower the heat and simmer for 1 1/4 hours. Strain the stock.
The stock can be refrigerated for 3 days or frozen for 1 month.