Peel and finely chop the onion. Put it with the olive oil in a soup pot over medium-high heat. Sauté, stirring occasionally, until the onion turns a rich golden color, about 5 minutes.
While the onion is sautéing, cut the zucchini into ½-inch dice. Peel the carrots and cut into ½-inch dice. Snap or cut off the ends of the green beans and cut into ½-inch lengths