Lemony Chicken and Orzo Soup

INGREDIENTS

• 1 olive oil
• 1 medium leek, white and pale-green parts only, halved lengthwise, sliced crosswise 1/2-inch thick
• 1 celery stalk, sliced crosswise 1/2-inch thick
• 12 skinless, boneless chicken thighs
• 6 low-sodium chicken broth
• Kosher salt, freshly ground pepper
• 1/2 orzo
• 1/4 chopped fresh dill
• Lemon halves (for serving)