Editor's note: The recipe and introductory text below are from Aroma: The Magic of Essential Oils in Food and Fragrance by Mandy Aftel and Daniel Patterson.

　　This is an incredibly useful and versatile stock. It has a sweet vegetal flavor that can be used in many different ways—as a base for sauces and soups, adding richness and depth without changing the essential flavor of what it is paired with. It keeps in the refrigerator for one week, or you can freeze it indefinitely.

Ingredients

　　3 medium yellow onions, sliced (about 22 ounces unpeeled)

　　1 yellow onion charred (see Chef's Tip, below)

　　4 carrots, peeled and sliced

　　1 fennel bulb, sliced

　　2 leeks, white and light green parts only, rinsed and sliced

　　1 small head celery root, peeled and sliced

　　1/2 bunch fresh thyme

　　4 quarts water

　　Put all ingredients in a nonreactive stockpot. Bring to a boil and simmer slowly for 2 to 3 hours. Strain through a fine mesh sieve. The stock should taste concentrated and sweet. Cool and refrigerate.

　　Chef's Tip:

　　Charring Onions: There are several recipes, especially for stocks, that call for charred onions, which add complexity and subtle smokiness. If you have gas burners, then you can cut peeled yellow onions in half and burn the cut side over an open flame for a few minutes, until mostly blackened. If you have electric burners, you can either put the cut side of the onions directly on burners set to high, or put a piece of aluminum foil on the burner first, and then char the onion on top of the aluminum foil.

　　Excerpted from Aroma (Artisan Books) Copyright 2004 by Mandy Aftel and Daniel Patterson