Preheat the grill or broiler. Meanwhile prepare the stuffing. In a saute pan, melt the butter and saute the shallots for 5 to 8 minutes, until they start to soften. Add the mushrooms, season with salt and pepper. Cook them until they become tender, about 5 minutes. Let cool and set aside. Meanwhile, prepare the marinade. In a bowl mix together the soy sauce and sherry, and season with salt and pepper. Place meat in a glass baking dish, pour in marinade over the meat and marinate for 30 minutes on each side. Transfer the meat to a flat surface, reserving the marinade, and spread the stuffing over the meat. Roll the meat, lengthwise, over the filling and tie it into place with twine. Brush with the reserved marinade and grill or place under the broiler on both sides for about 30 minutes or until desired doneness.