Preheat oven to 350 degrees.
In a small bowl, mix together mayo, horseradish, and cranberry sauce until just combined. Season with salt and set aside.
Place the stuffing patty on a lightly buttered Holy Sheet (or another sheet pan). Cover the stuffing with sliced turkey breast, shredded dark meat, and gravy. Warm through in the oven for 10 minutes.Very lightly toast the bread — it shouldn’t brown, just firm up a bit to hold the ingredients.
Gently warm the sweet potato mash in Saucy (or another saucepan).
To assemble, spread the horseradish-cranberry mayo on both slices of bread. Place a few slices of onion on the bottom piece of bread, top with the stuffing and turkey mound, add the mashed sweet potato, and finish with the top slice of bread.