Melt 1/2 cup butter in a Dutch oven over medium-high heat; add diced onion, and sauté 5 minutes or until tender.
Whisk in flour until smooth, and cook, whisking constantly, 1 minute. Gradually add milk and wine; cook, whisking constantly, 5 to 6 minutes or until thickened.
Add bouillon granules, seasoned pepper, and 1 cup cheese. Cook, whisking constantly, 1 minute or until bouillon granules dissolve and cheese melts. Remove from heat. Stir in chicken, pasta, and mushrooms. Spoon into a lightly greased 13- x 9-inch baking dish.
Bake chicken mixture, covered, at 350° for 20 minutes.
Stir together remaining 1/2 cup cheese, breadcrumbs, and 2 tablespoons melted butter, and sprinkle evenly over casserole. Bake casserole 10 to 15 more minutes.