Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin pan.
In a large bowl, combine the ground turkey, stuffing mix, egg, milk, onion, dried sage, and black pepper. Mix gently until just combined. Let the mixture stand for 5 minutes so the stuffing mix can absorb some of the moisture.
Divide the turkey mixture among the prepared muffin cups, pressing it in lightly. Make a small indentation in the center of each portion and add a little cranberry sauce.
Bake for 20 to 25 minutes, until the meatloaf muffins are cooked through and reach 165°F in the center.
While the muffins bake, spoon the mashed potatoes into a piping bag or a sturdy resealable bag with a corner snipped off.
Remove the pan from the oven. Pipe a crown of mashed potatoes onto each muffin and brush the tops lightly with melted butter.
Return the pan to the oven and bake for 8 to 10 minutes, until the potato crowns are lightly golden. Let the muffins rest for 5 minutes before carefully removing them from the pan.