To make the vegetable stock, add the olive oil to a heavy-bottomed pan or stockpot over a medium heat.
Add the onion, leek, carrot and fennel and sweat for 2-3 minutes.
Add enough cold water to generously cover the vegetables and turn up the heat to high.
Add the garlic, peppercorns, mushrooms, celery, tomatoes and parsley and stir together. Bring to the boil and boil gently for 15 minutes.
Pour the stock through a sieve. Discard the vegetable pieces or reserve for another use. The liquid stock is ready to be used. It can be stored in the fridge for up to three days or frozen in batches for future use.