Heat 1 tablespoon olive oil in a medium soup pot over medium heat. Add the chopped leek, zucchini, fennel, and celery and cook, stirring occasionally until the vegetables begin to caramelize lightly, about 15 minutes. Season with salt and pepper to taste. Add the ground fennel seeds, stir, and pour in the stock. Bring to a boil, reduce the heat, cover and simmer for 20-25 minutes, until the vegetables are very soft.