Cut poached chicken breast into 1-inch chunks. Place in a medium bowl and add lemon juice. Toss to combine.
Add mayonnaise, tarragon, and salt and pepper to chicken mixture. Stir to combine.
Cut bread into 1/2-inch slices, lengthwise. Place a scoop of chicken mixture between two pieces of bread. Press firmly on sandwich to flatten. Slice sandwich in half or in quarters.
(c)2003 Martha Stewart Living Omnimedia, Inc. All Rights Reserved.