This simple sandwich is Martha's favorite way to enjoy leftover Thanksgiving turkey.
In a small bowl, mix together mayonnaise, ketchup, and cornichons until well combined; set aside.
Spread one side of each slice of bread with butter. Top buttered side of two slices of bread with mustard. Top each with 3 slices turkey and season with salt and pepper. Spread a quarter of the mayonnaise mixture over turkey and top each with 3 slices of tomato, 3 pieces of bacon, and 2 radicchio leaves. Evenly divide watercress between each.
Evenly divide remaining mayonnaise mixture between buttered side of remaining two slices of bread; place on top of watercress to form sandwich. Serve with cranberry sauce, if desired.