The following ingredients are the backbone of a basic vegetable stock: onions, carrots, celery, thyme, parsley, bay leaf, garlic, leek trimmings, salt, onion for the initial sautéing of vegetables.
In making a soup that’s not at the sweet end of the flavor spectrum, it’s always good to include some of these: chard stems and leaves; fresh mushrooms; the soaking water from dried mushrooms; scallions, in addition to or in place of onions or leeks; potato parings; celery root (well scrubbed); parsley root; Jerusalem artichoke; lettuce; eggplant. In winter stocks, try these: dried sage (about 1 or 2 teaspoons per 2 quarts of water); dried rosemary (about a 1-inch piece); caramelized onions; mushrooms fresh or dried.
For a versatile basic stock, I recommend avoiding vegetables in the cabbage family: no turnips or rutabagas, no broccoli or cauliflower. Also no red beets (save them for beet soup), no onion skins, no tiny celery seeds or powdered herbs or ground pepper, for they can make stock bitter. It’s also good to avoid spiking your stock with excessive amounts of greens—use no more than 4 cups. And please, don’t add anything you wouldn’t eat otherwise. No spoiled vegetables or funky foods. But one exception to that last note is Parmesan cheese rinds—whole chunks may be added to stocks that will be used for soups with beans. When in doubt about an ingredient, try this simple trick: Simmer it alone for a while, then taste the water to see if the flavor is to your liking. Unlike meat stocks, vegetable stocks don’t benefit from hours of cooking.
Start by heating the oil in a soup pot; choose a sturdy one with a good handle. Using vegetables that have been chopped into roughly 1-inch chunks, sauté with garlic and herbs for 5 to 10 minutes, stirring frequently. Add 2 teaspoons salt and 2 quarts cold water and bring to a boil. Lower the heat and simmer uncovered for 30 to 45 minutes, no more, for when the vegetables have given up their flavors, they have nothing more to offer. Strain and store covered; refrigerate if keeping for more than an hour or two.
Once strained, the flavor of stocks can be concentrated by boiling them, uncovered, until they are reduced by half. This will make storing the basic vegetable stock a little easier.