Heat oil in a stockpot over medium heat until hot. Add onions, celery and garlic. Cook and stir 5 minutes or until tender. Add leeks and carrots; cook and stir 5 minutes. Add water, mushrooms, parsley, thyme, salt, peppercorns and bay leaf; bring to a boil. Reduce heat; simmer, uncovered, 1 hour. , Remove from heat. Strain through a cheesecloth-lined colander; discard vegetables. If using immediately, skim fat. Or, refrigerate 8 hours or overnight; remove fat from surface. Broth can be covered and refrigerated up to 3 days or frozen up to 6 months.