Combine the cold mashed potatoes, cold stuffing, egg, cheese, parsley or chives, and black pepper in a large bowl. Mix just until evenly combined, taking care not to overmix.

Refrigerate the mixture for 20–30 minutes. This firms the potatoes and stuffing so the balls are easier to shape and less likely to fall apart.

Heat the oven to 400°F. Line a baking sheet with parchment paper and lightly coat it with cooking spray.

Scoop approximately 1/4 cup of mixture for each ball. Compress each portion firmly with your hands and shape it into a round ball, making about 12–14 balls.

Roll each ball in the panko breadcrumbs and arrange the coated balls on the prepared baking sheet. Spray or lightly brush the tops with cooking spray or vegetable oil.

Bake for 20–25 minutes, turning the balls once halfway through the cooking time. They are ready when the coating is golden and the centers are heated through.

Serve the balls hot with turkey gravy, additional leftover gravy, or cranberry sauce alongside. For a fried version, heat about 1 inch of neutral oil to 350°F and fry in batches for approximately 3–4 minutes, turning carefully, then drain on a paper-towel-lined plate.