Cut 12 thin slices from 1 zucchini. Chop remaining zucchini (about 4 cups). In large Dutch oven, heat oil over medium heat until hot. Add leek; cook about 3 minutes or until starting to soften. Add chopped zucchini and potato; cook 1 minute. Stir in vegetable broth and pepper. Heat to simmering; cook 15 to 20 minutes or until potatoes are cooked through. Stir in spinach leaves, and cook until just starting to wilt.