Spread the mustard on both slices of bread, then top each slice of bread with cheese. On one slice, top with turkey slices, mashed potatoes, cranberry sauce, and stuffing. Drizzle with gravy and carefully top with the remaining slice of bread.
Spread the top side of the bread with 1 tablespoon of butter. Invert the sandwich, butter-side down, onto a hot panini maker (or a grill pan or skillet over medium-low heat). Spread the top with the remaining 1 tablespoon butter.
Close the panini maker and grill the sandwich until the bread is crusty and golden, the fillings are hot, and the cheese is melted, about 6-8 minutes. Remove, slice in half and serve immediately.