Place all ingredients into a 6-quart stockpot, bring to a boil. Lower the heat to warm setting and let simmer for 1 1/2 hours, covered.
Let the broth cool until it's okay to handle.
Strain the broth through a strainer into a large bowl.
This will keep in the fridge in a tightly sealed container for up to a week, or freeze for up to 5 months.

1 large onion, skin on, coarsely chopped
2 large carrots, coarsely chopped
2 leeks, rinsed, coarsely chopped
8 oz (226g) whole white mushrooms
3 cloves garlic, smashed
1 tablespoon fresh parsley
1 bay leaf
1/4 teaspoon salt
9 cup water, or enough to cover all ingredients