Heat oil in a medium Dutch oven over medium. Add leeks; cook, stirring often, until softened, about 6 minutes. Stir in zucchini; cook, stirring occasionally, until zucchini is just tender but not browned, about 4 minutes. Add chicken stock, and increase heat to medium-high. Bring to a boil; reduce heat to medium-low, and simmer 8 minutes.