Place potatoes in a medium saucepan; cover with cold water, and bring to a boil. Reduce heat; simmer until just tender, about 7 minutes. Drain; set aside.
Heat 3 tablespoons butter in a large non-stick skillet over medium heat. Add apple; cook until golden, about 11 minutes. Transfer to a plate; set aside.
Add remaining 2 tablespoons butter, the onion, and celery to skillet; cook until slightly softened, 9 to 10 minutes. Add wine; cook until liquid has almost evaporated. Add turkey, potatoes, cream, and stock. Simmer until sauce is creamy and vegetables are tender, about 12 minutes. Stir in sage and apple; season with salt and pepper. Spoon onto buttered toast, and drizzle with sauce.