3 tablespoons extra-virgin olive oil
2 1/2 tablespoons apple cider vinegar
1 1/2 tablespoons coarsely chopped fresh tarragon
2 teaspoons fresh lemon juice
1/4 teaspoon sugar
3 celery stalks, thinly sliced diagonally, plus 1/4 cup loosely packed celery leaves
2 small fennel bulbs, thinly sliced crosswise,
plus 1 tablespoon chopped fennel fronds
1 firm, crisp apple (such as Pink Lady, Gala, or Granny Smith), julienned
Kosher salt and freshly ground black pepper
Whisk first 5 ingredients in a medium bowl.
Add celery and celery leaves, thinly sliced fennel and chopped fennel fronds, and apple; toss to coat.
Season to taste with salt and pepper.