For dressing

• 1/4 cup whole-milk yogurt
• 1 tablespoon extra-virgin olive oil
• 1 tablespoon fresh lemon juice
• 1 tablespoon minced shallot
• 1 tablespoon finely chopped fresh chives
• 1 teaspoon sugar
• 1/2 teaspoon Dijon mustard
• 1/4 teaspoon salt

For salad

• 1/2 bunch sorrel, coarse stems discarded and leaves torn into bite-size pieces (4 cups)
• 2 chicken breasts, grilled and cut into bite-sized pieces

• 1/2 pound hearts of romaine, torn into bite-size pieces (4 cups)
• 1/4 pound frisée, trimmed and torn into bite-size pieces (2 cups)
• 1/2 cup loosely packed fresh flat-leaf parsley
• 2 tablespoons loosely packed fresh tarragon, leaves coarsely chopped if large

Make dressing:
Whisk together all dressing ingredients in a large bowl.

Make salad:
Toss together all salad ingredients with dressing in bowl. Season with salt and pepper.