Chop scrubbed vegetables into 1-inch chunks. Remember, the greater the surface area, the more quickly vegetables will yield their flavor.
Heat oil in a soup pot. Add onion, celery, carrots, scallions, garlic, parsley, thyme, and bay leaves.
Cook over high heat for 5 to 10 minutes, stirring frequently.
Add salt and water and bring to a boil. Lower heat and simmer, uncovered, for 30 minutes. Strain. Discard vegetables.
Other ingredients to consider: mushrooms, eggplant, asparagus (butt ends), corn cobs, fennel (stalks and trimmings), bell peppers, pea pods, chard (stems and leaves), celery root parings, marjoram (stems and leaves), basil, potato parings . . . Get the idea?