Steam beans for 3 to 5 minutes, or until tender-crisp. Rinse under cold water. Place them in a salad bowl.
Trim fennel leaves, reserving 2 Tbsp. minced leaves. Slice the stalks. Quarter bulb lengthwise, remove core and thinly slice. Add to the salad bowl along with mushrooms and lemon zest. Whisk vinegar, water, lemon juice and oil in a small bowl until well blended. Pour the dressing over the salad and add the reserved fennel leaves. Toss lightly. Season with salt and pepper. Cover and chill for 30 minutes to 1 hour.