Spray the bottom of a slow-cooker with cooking spray.
Cut a cone-shaped piece around the stem at the top of each apple, about 2 inches wide, and remove it. Use a melon baller to remove the core and seeds, leaving the bottom and about ½ an inch of the sides of the apple intact.
In a medium bowl, combine cranberries, walnuts, ginger, brown sugar, and margarine. Mix together well. Divide the filling between the apples, pressing down to fill them completely. Bake in the slow-cooker set on high for 3 hours.