1 tbsp olive oil
- 1 medium carrot, largely diced
- 1 medium leek, halved lengthwise, rinsed, and sliced crosswise into 1-inch pieces (white and pale green parts only)
- 1 medium yellow onion, largely diced
- 6 medium garlic cloves
- 2 lbs medium white button mushrooms, stems trimmed and quartered
- 8 Italian parsley sprigs
- 6 thyme sprigs
- 1 bay leaf
- 2 tsp whole black peppercorns
- 12 cups water
- 1/4 tsp sea salt
Heat the oil in a large pot over medium-high heat until shimmering. Add the carrot, leek, and onion and cook, stirring occasionally, until the onions and leeks have softened, about 8 minutes. Add the garlic and cook, stirring occasionally, until fragrant, about 1 minute.
- Add the mushrooms, parsley, thyme, bay leaf, and peppercorns and cook, stirring occasionally, until the mushrooms start to release some moisture, about 4 minutes.
- Add the water and bring to a boil. Reduce the heat to low and simmer until the vegetables are completely soft and the stock has a pronounced mushroom flavour, about 1 hour.
- Remove from the heat and strain through a fine-mesh strainer set over a large heatproof container or saucepan; discard the contents of the strainer. Stir in the salt. If not using immediately, let cool to room temperature, then refrigerate for up to 3 days or freeze for up to 1 month.