Preparation Combine lemon juice, lemon peel and mustard in bowl; gradually whisk in oil. Stir in basil and onion. Season with salt and pepper. Let stand at least 1 hour and up to 6 hours, stirring occasionally. Steam zucchini until just crisp-tender, about 5 minutes. Transfer to large bowl; cool completely. Add dressing and toss to coat. Let stand 1 hour for flavors to develop, stirring often.
Preparation Combine lemon juice, lemon peel and mustard in bowl; gradually whisk in oil. Stir in basil and onion. Season with salt and pepper. Let stand at least 1 hour and up to 6 hours, stirring occasionally. Steam zucchini until just crisp-tender, about 5 minutes. Transfer to large bowl; cool completely. Add dressing and toss to coat. Let stand 1 hour for flavors to develop, stirring often.