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.