In 6-quart saucepot, cook pasta as label directs; drain. Return to saucepot; keep warm.
Meanwhile, in 2-quart saucepan over medium heat, in 3 tablespoon hot butter, cook onion until tender, stirring occasionally. Stir in flour until blended; cook 1 minute. Gradually stir in milk, mushrooms, bouillon, salt, and pepper; cook, stirring constantly, until mixture thickens slightly. Remove saucepan from heat; stir in Parmesan cheese.
Cut bread into small cubes to make about 2 cups coarse breadcrumbs. In small saucepan over low heat, melt 3 tablespoons butter; remove saucepan from heat; stir in breadcrumbs and set aside.
Preheat oven to 350°F. To pasta in saucepot, add sauce mixture and turkey; gently toss to mix well. Spoon mixture into 12" by 8" baking dish; top with breadcrumbs. Bake 20 minutes or until heated through.