Sauté onion, fennel, and garlic in hot olive oil in a large Dutch oven over medium-high heat 5 minutes; add zucchini, and sauté 3 minutes. Stir in vegetable broth and water; bring to a simmer. Cook 15 minutes or until vegetables are tender. Remove from heat. Process with a handheld blender 4 minutes or until smooth. Stir in buttermilk, salt, vinegar, lemon zest, and lemon juice. Let stand at room temperature 15 minutes. Cover and chill 2 hours or up to 2 days.