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.