Preparation: Rinse and chop all the vegetables. There's no need to peel them unless you prefer to.
Sauté Aromatics: In a large pot, sauté the onion, carrots, celery, and garlic over medium heat for about 5 minutes until they start to soften. This step enhances the flavor of the broth.
Add Remaining Ingredients: Add the parsnip, leek (if using), water, mushrooms, tomato, parsley, bay leaves, and black peppercorns to the pot.
Bring to a Boil: Increase the heat to high and bring the mixture to a boil.
Simmer: Once boiling, reduce the heat to low and let the broth simmer uncovered for at least 45 minutes to 1 hour. This allows the flavors to meld.
Strain: After simmering, strain the broth using a fine-mesh strainer or cheesecloth into a large bowl or another pot. Discard the solid vegetable pieces.
Season: Taste the broth and add salt as needed. Remember that you can always add more salt later, so start with a smaller amount.
Cool and Store: Allow the broth to cool before transferring it to containers. Refrigerate or freeze for later use.