In a small mixing bowl, combine the shallot, vinegar, lemon juice, and mustard and whisk to combine. Slowly add the oil, whisking to incorporate. Add the yogurt, salt and pepper and blend well. Taste for seasoning. Combine all the salad ingredients in a large salad bowl. Add the dressing and toss until all the vegetables are well coated. Sprinkle with freshly ground pepper, if desired, and serve.
ADVANCED PREPARATION: The dressing can be prepared up to 1 day ahead and refrigerated. Whisk the dressing well before using. The salad can be prepared up to 2 hours ahead and refrigerated.
Recipe courtesy of Diane Worthington, "The California Cook"