Stabilize your grill for a direct cook at 450°F. For this particular cook I prefer a soapstone but you could use the cast iron griddle.
Place your turkey on the soapstone and splash a bit of water on it to refresh and steam the meat. Butter the sliced bun and toast each side on the soapstone. Warm the cooked bacon and caramelized onions.
Stack the turkey to the width of the bun, top the warmed turkey with reheated onions and top with the 2 slices of provolone.
Transfer warmed mashed potatoes to the center of the presentation plate.
Spread the cranberry sauce on the toasted buns and top the bottom bun with the turkey, onion and cheese stack. Top with the top bun and stab through with the sprigs of rosemary. Place the sandwich on top of the mound of mashed potatoes and pour the gravy over the top of the entire sandwich.