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Heat oil in a large saucepan over medium heat. Add onion, carrot, and celery and cook, stirring occasionally, until softened, 5-7 minutes. Add cremini mushrooms, thyme, parsley, peppercorns, salt, and 7 cups cold water, bring to a boil and reduce to a simmer and cook for 30 minutes. Remove from the heat and let sit for 30 minutes longer.
Strain stock into a clean saucepan and keep warm over low heat (you should have about 6 cups).