In a large skillet, cover the chicken, lemon, halved celery, and ½ teaspoon pepper with salted water. Bring to a simmer and cook until the chicken is cooked through, 4 to 5 minutes.
Remove the chicken and shred, using 2 forks. Discard the lemon and halved celery.
Toss the chicken with the chopped celery, celery leaves, shallot, mayonnaise, almonds, raisins, and ¼ teaspoon pepper in a medium bowl. Serve over the lettuce with the baguette.