Remove and discard the tough outer layers of the leeks.

Cut the leeks open and in half lengthwise.

Wash the leeks very well to remove any sand, then dry with paper towels.

Cut the leeks into 1-inch pieces across the leek stalks.

Soak the dried black mushrooms in hot water for 20 minutes, or until soft.

Drain the mushrooms.

Rinse the mushrooms in warm water and drain again.

Squeeze out any excess moisture from the mushrooms.

Remove and discard the stems of the mushrooms.

Heat a large stockpot and add the vegetable oil.

Add the leeks and stir-fry for 1 minute.

Add 4 cups of water to the pot, bring to a boil, then add the black mushrooms, soybean sprouts, carrot, salt, and white pepper.

Return the broth to boiling again, then turn the heat down to medium-low.

Cook, covered, for 1 hour.

Strain the broth to remove the solids.

Cool the broth and store in the refrigerator for up to 5 days or freeze in a tightly sealed container for up to 3 months.