Step 1Heat the oil in a 6-quart pressure cooker over medium-high heat. Add the leeks, garlic and onion. Cook until the leeks are tender and the onion begins to brown, about 5 minutes. Scrape the caramelized crust from the pan with a wooden spoon, if necessary. Add the carrots, celery and parsnips and cook 5 minutes more.
Step 2Add the squash, potatoes, mushrooms, tomato, bay leaves, parsley, peppercorns and water.
Step 3Lock the lid in place and bring the pressure cooker to high pressure. Reduce the heat to maintain high pressure and cook 12 minutes. Release the pressure naturally, if time permits.
Step 4Strain the stock through a strainer, pressing the vegetables against the side to release as much flavor and liquid as possible. Discard the solids. Season to taste with salt, if needed.