Lemony chicken and orzo soup recipe

Chicken second courses

1 tablespoon olive oil 1 medium leek, white and pale-green parts only, halved lengthwise, sliced crosswise ½-inch thick 1 celery stalk, sliced crosswise ½-inch thick 12 ounces skinless, boneless chicken thighs 6 cups low-sodium chicken broth Kosher salt, freshly ground pepper ½ cup orzo ¼ cup chopped fresh dill Lemon halves (for serving)