Heat the olive oil in a large stockpot over medium heat. Add the onions, carrots, and celery and cook for 8 minutes, stirring occasionally, until the edges begin to color.

Add the mushrooms and garlic. Cook for 5 minutes, stirring, until the mushrooms release some moisture and the vegetables smell deeply savory.

Add the bay leaves, parsley, peppercorns, water, and salt. Bring just to a boil, then lower the heat to a gentle uncovered simmer.

Simmer for 40 to 45 minutes. Avoid a hard boil, which can make the stock cloudy and drive off liquid too quickly.

Strain through a fine-mesh sieve without pressing hard on the vegetables. Taste and add salt only if the stock needs it. Cool promptly before storing.