Sauté turkey, mushrooms and onion in oil until turkey is cooked and onion tender; break turkey into small pieces.  Add pistachios, oregano, salt and pepper; mix well.  Roll each sheet of pastry to 16 x 12-inches; cut into 12 squares (4 x 4-inches each).  Place about 1 rounded tablespoon filling on one side of pastry square; moisten edges with water.  Fold in half to enclose filling; press firmly with fork to seal.  Place on baking sheet.  Beat egg yolk and water together; brush over pastries.  Bake at 400° F.  20 to 25 minutes or until golden.  Serve hot.