2 tablespoons fresh lemon juice
1 teaspoon grated lemon peel
1 teaspoon Dijon mustard
5 tablespoons olive oil
1/2 cup thinly sliced fresh basil
1/2 cup paper-thin slices red onion
1 1/2 pounds medium zucchini, trimmed, quartered lengthwise, then cut crosswise in half
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.
Makes 6 to 8 servings.