Combine rice and water in pan and bring to a boil. Cover and cook over low heat for 40 to 45 minutes or until rice is tender. Drain off excess water. Melt butter in large kettle. Add onions, celery and mushrooms. Cook until vegetables are crisp-tender, about 5 minutes. Add water if necessary to prevent scorching. In a blender container, combine 1 cup cooked rice and 1 cup hot chicken stock. Blend until completely smooth. Add to the vegetable mixture along with remaining chicken stock, rice and chopped turkey. Cook for 30 minutes over medium heat, stirring often to prevent sticking. Add the milk, salt and pepper just before serving.