Celery, Apple, and Fennel Slaw

INGREDIENTS

• 1 firm, crisp apple (such as Pink Lady, Gala, or Granny Smith), julienned
• 3 tablespoons extra-virgin olive oil
• Kosher salt and freshly ground black pepper
• 3 celery stalks, thinly sliced diagonally, plus 1/4 cup loosely packed celery leaves
• 2 teaspoons fresh lemon juice
• 2 1/2 tablespoons apple cider vinegar
• 1 1/2 tablespoons coarsely chopped fresh tarragon
• 2 small fennel bulbs, thinly sliced crosswise, plus 1 tablespoon chopped fennel fronds
• 1/4 teaspoon sugar