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