Method:
Fill a medium pot with water and bring to a boil over high heat. Add chicken, cover and return to a boil. Simmer chicken for 2 to 3 minutes, then turn off the heat and let the chicken steep, covered, until no longer pink in the thickest part of the breast, 8 to 12 minutes. Using a slotted spoon, transfer chicken to a plate and let cool for approximately 20 minutes. (The chicken can be prepared a day ahead of the rest of the salad, if desired.)
In a medium bowl combine celery, mayonnaise, mustard, onion, almonds, tarragon, lemon juice and pepper and stir to blend. Cut chicken into 1/2-inch cubes, then fold it into the mayonnaise mixture. Arrange 6 slices of bread on a work surface. Divide the chicken salad equally among the bread slices. Top each with a lettuce leaf, slice of tomato and second slice of bread. Cut sandwiches in half and serve.