Prepare stuffing according to package directions. Meanwhile, in a large skillet, saute mushrooms and onion in butter until tender. Add garlic; cook 1 minute longer.

In a large bowl, combine the chicken, corn, broccoli, soup, milk, sour cream, mayonnaise, pepper and mushroom mixture; transfer to a greased 13-in x 9-in. baking dish. Top with stuffing. Cover and refrigerate overnight.

Remove from the refrigerator 30 minutes before baking. Cover and bake at 350° for 35 minutes. Uncover and bake 10-15 minutes longer or until stuffing is lightly browned. Let stand for 10 minutes before serving.