Pulse the leeks, carrots, celery root, and parsley in a food processor until the vegetables are finely and evenly minced, stopping to scrape down the bowl as needed.

Add the dried onion, salt, tomato paste, and soy sauce to the processor. Process until a thick, uniform paste forms; the salt helps break down the vegetable fibers into a smooth concentrate.

Transfer the base to an airtight container and store in the freezer. The high salt content prevents the mixture from freezing solid, so it should remain soft enough to scoop directly with a spoon.

Whisk the base into boiling water until fully dissolved to create a clear, savory broth, or stir it directly into sauces and stews to deepen the flavor. Serve hot.