Preheat oven to 375 degrees F.
Heat oil in a large skillet. Saute onions, mushrooms, and cranberries until soft. Season with salt and pepper. Add cornbread and continue to cook until they are soft and fully incorporated. Cool.
Roll out one sheet of pastry one inch wider than turkey breast on all four sides. Place turkey in center of pastry. Using clean hands, mold the cranberry-cornbread mixture around top and sides of turkey. Top with second piece of pastry, pressing it tight to the turkey on all sides. With fingers, press seams closed, cut off excess pastry, and brush with egg.
Bake for 45 minutes to one hour or until pastry is browned and turkey is fully cooked through. Cool and slice in three pieces.