Poach chicken breasts in chicken broth and sherry for 15 minutes or until tender. Cool and drain. Save chicken broth for another use or discard.
Split chicken breasts in half lengthwise and wrap each piece in a thin slice of ham. Coat ham with a thin layer of mustard. Sprinkle with tarragon and then wrap breast in slice of cheese.
. Place thawed puff pastry shells on lightly floured surface and roll out each onto into 4" circle. Put each chicken breast half on pastry circle. Position it so that you can fold it over to cover breast, making half moon shape. Wet pastry edges with water, crimp and seal. Put on ungreased baking sheet in preheat 400 oven and bake for 20 minutes or until puffed and golden.
. Transfer to warmed serving platter and serve at once. may also be served cold. Serves 6