Combine first 5 ingredients in a medium bowl; stir well with a whisk.
Melt butter in a large nonstick skillet over medium-high heat. Add mushrooms and next 3 ingredients; cook 6 minutes or until tender. Add milk mixture, and cook 6 minutes or until thick and bubbly, stirring constantly. Add Parmesan cheese and sherry; stir well. Remove from heat; add noodles, and stir gently.
Place bread in a food processor; pulse 10 times or until coarse crumbs form to measure 1/3 cup. Spoon mixture into a 10-inch gratin dish; sprinkle with breadcrumbs. Bake at 450° for 10 minutes. Let stand 10 minutes.