Spread the mustard on both slices of bread, then lay a slice of cheese on each piece. On one slice, arrange the turkey and the cranberry sauce. On the other slice, lay on the dressing and spoon the gravy over the top.
Carefully unite the two halves into one sandwich, then spread the top side of the bread with 1 tablespoon of butter.
Invert the sandwich, butter-side down onto a grill pan or skillet over medium-low heat. Spread the top with the remaining 1 tablespoon of butter.
Grill the sandwich until the bread is crusty and golden, the fillings are hot, and the cheese is melted. Flip the sandwich halfway through to grill the other side.
Pull it off the heat and slice it in half.