Melt butter in a large skillet over medium heat. Place bread slices in a single layer and toast until golden brown on both sides. Work in batches if needed, adding more butter as necessary. Transfer toasted bread to a plate.
Optional: Heat turkey slices in the microwave in 30-second increments until warmed through, or use cold.
Build sandwiches by spreading mustard on one side of the toasted bread.
Layer 3-4 turkey slices and top with Swiss cheese.
Add a generous portion of stuffing and drizzle with warm gravy.
Top with fresh arugula and cranberry sauce.
Close the sandwich with the remaining bread slice and press gently.
Serve immediately with chips and extra warm gravy on the side.
Notes
Toast the bread until golden brown for the best texture and stability
Warm the gravy separately before adding to prevent soggy bread
Layer cheese next to warm turkey to encourage melting
Don't oversaturate with gravy - you can always serve extra on the side
Build sandwiches just before eating for the best results