If using chicken breast, place it in a medium-size saucepan with water, onion, celery tops, 1/2 teaspoon salt, and a dash of pepper.
Simmer, covered, 20 to 30 minutes, or until chicken is tender.
Let stand until cool enough to handle, then skin chicken and take meat from bones.
Dice chicken (you should have about 1 cup).
Combine chicken, celery, and almonds in medium-size bowl.
Mix mayonnaise, milk, 1/4 teaspoon salt, mustard, and a dash of pepper; stir into chicken mixture, tossing lightly to mix.
If making 2 portions, chill 1 portion on a plate until serving time; place remainder in a covered refrigerator container in coldest part of refrigerator for tomorrow's meal.
At mealtime, split Vienna roll and butter it.
Line buttered roll with lettuce; fill with salad.
Garnish with cherry tomatoes.