Preheat oven to 325 degrees F (165 degrees C). Grease a 9x13-inch baking dish.
With a sharp knife, slice a deep cut horizontally into the thick side of the turkey breast, leaving a deep pocket. Mix together the spinach, brown rice, cream cheese, pine nuts, allspice, salt, and black pepper, until well combined. If stuffing doesn't hold together, mix in a little more cream cheese.
Place turkey into the prepared baking dish, and stuff the rice mixture into the pocket of the turkey breast. Use skewers to hold the turkey breast closed. Top the meat with thin pats of butter; cover the baking dish with aluminum foil.
Bake in the preheated oven until the turkey is no longer pink and the stuffing is hot, about 1 hour. Remove foil, and return turkey breast to the oven until well browned, about 45 more minutes. Baste occasionally with pan juices. An instant-read meat thermometer inserted into the thickest part of the breast, in the stuffing, should read 165 degrees F (75 degrees C). Slice across the grain of the meat to serve.