1. Preheat oven to 375°. Heat 2 tablespoons butter in a large, deep, heavy skillet over medium heat. Add mushrooms and green onions, and sauté 8 to 10 minutes or until soft. Transfer to a large plate.
2. Melt 2 more tablespoons butter in skillet, and add flour, stirring until smooth, about 1 minute. Add chicken broth and half-and-half. Cook, stirring occasionally, 6 to 8 minutes or until lightly thickened and smooth. Remove from heat; stir in 1/4 cup Parmesan and next 3 ingredients. Fold in mushroom mixture, cooked pasta, and turkey. Return to medium heat, and cook until thoroughly heated.
3. Transfer to a lightly greased 11- x 7-inch (2-quart) baking dish. Melt remaining 1 tablespoon butter; mix in breadcrumbs and remaining 1 tablespoon Parmesan. Sprinkle topping evenly over dish. Place on middle oven rack, and bake 25 to 30 minutes or until bubbling and beginning to brown.