Preparation Bring large saucepan of salted water to boil. Add chicken breasts; reduce heat to medium-low, cover and simmer until chicken is just cooked through, about 12 minutes. Transfer chicken to plate; cool. Mix celery, 1/2 cup mayonnaise, onion, tarragon, lemon juice, and lemon peel in large bowl to blend. Cut chicken into 1/2-inch cubes; stir into mayonnaise mixture. Season with salt and pepper. (Can be made 4 hours ahead. Cover; chill.) Arrange 6 bread slices on work surface. Spread with 3 tablespoons mayonnaise. Divide salad among bread slices. Top each with lettuce and second bread slice. Cut sandwiches in half and serve.
Preparation Bring large saucepan of salted water to boil. Add chicken breasts; reduce heat to medium-low, cover and simmer until chicken is just cooked through, about 12 minutes. Transfer chicken to plate; cool. Mix celery, 1/2 cup mayonnaise, onion, tarragon, lemon juice, and lemon peel in large bowl to blend. Cut chicken into 1/2-inch cubes; stir into mayonnaise mixture. Season with salt and pepper. (Can be made 4 hours ahead. Cover; chill.) Arrange 6 bread slices on work surface. Spread with 3 tablespoons mayonnaise. Divide salad among bread slices. Top each with lettuce and second bread slice. Cut sandwiches in half and serve.