Preheat oven to 375°. In a saucepan, mix first 4 ingredients until smooth; gradually stir in milk. Bring to a boil over medium heat; cook and stir until thickened, about 2 minutes. Add vegetables and turkey; cook and stir until heated through. Transfer to a greased 8-in. square baking pan. , Unroll crescent dough and separate into 8 triangles; arrange over turkey mixture. Bake until casserole is heated through and topping is golden brown, 15-20 minutes.