Preheat your oven to 375°F (190°C). Line a 13 x 18-inch (33 x 46 cm) rimmed baking sheet with parchment paper. Make sure the oven has reached the proper temperature before baking so the outside of the stuffing balls begins to firm up evenly.
Place 1 cup (about 140 g) finely chopped or shredded cooked turkey and 2 cups (about 300 g) prepared, cooled stuffing in a large mixing bowl. Break apart any large clumps of stuffing with a spoon. The mixture should have small, evenly distributed pieces, with no large chunks that could make the balls difficult to shape.
Add 1 cup (about 210 g) cooled mashed potatoes to the turkey and stuffing. Fold everything together until the potatoes are evenly distributed. The mixture should look moist and slightly sticky, but the individual pieces of turkey and stuffing should still be visible.
Add 1 lightly beaten large egg (about 50 g without shell), 1/3 cup (about 35 g) plain dry breadcrumbs, 1/4 teaspoon (about 0.5 g) black pepper, and 1/2 teaspoon (about 1 g) dried rubbed sage. Stir until everything is evenly combined and the mixture holds together when pressed with a spoon. Don't overmix; you want a soft, textured mixture rather than a smooth paste.
Divide the mixture into 16 equal portions, using a 1/4-cup (60 mL) measuring cup as a rough guide. Gently compress each portion between your palms and roll it into a compact ball approximately 2 inches (5 cm) wide. Arrange the balls on the prepared baking sheet with about 1 inch (2.5 cm) of space between them. They should hold their shape without spreading or crumbling.
Place the baking sheet on the middle oven rack and bake at 375°F (190°C) for approximately 25 minutes. The stuffing balls should feel firm, develop lightly golden edges, and have a few toasted spots on top. Check the center of a ball with an instant-read thermometer and make sure it reaches 165°F (74°C). If necessary, continue baking until that temperature is reached.
Remove the baking sheet from the oven and let the turkey stuffing balls rest for 5 minutes. Their surfaces should remain lightly golden while the centers settle and become easier to handle. Gently lift them from the parchment paper with a spatula and serve while warm.