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 the butter.
Invert the sandwich, butter-side down, onto a hot panini maker (or a grill pan or skillet over medium-low heat; see Cook's Note). Spread the top with the remaining 1 tablespoon butter.
Close the panini maker and grill until the bread is crusty and golden, the fillings are hot, and the cheese is melted. (If using a grill pan or skillet, flip the sandwich halfway through to grill on the other side.)
Pull the sandwich off the heat and slice it in half--this sandwich is to die for!
If you use a grill pan or skillet, place a second heavy pan or skillet on top of the sandwich to press it.