Cook beans, covered, in a small amount of boiling salted water for 20 to 25 minutes or until tender. Drain. Cover and chill for 1 to 24 hours.
Meanwhile, for dressing, in a screw-top jar combine olive oil or salad oil, white wine vinegar, tarragon, water, mustard, and garlic. Cover and shake well. Chill up to 24 hours.
To serve, in a large salad bowl combine mixed greens, zucchini slices, tomato wedges, and cooked beans. Shake dressing well; pour about half of the dressing over salad. Toss lightly to coat. Cover and store remaining dressing in the refrigerator for up to 2 weeks. Makes 4 side-dish servings.