Preheat oven to 400°.
Remove sausage from casings. Place a large skillet coated with cooking spray over medium heat; cook sausage until browned, stirring to crumble. Drain well; set aside. Wipe drippings from pan with a paper towel.
Melt butter in pan over medium heat. Add flour; stir with a whisk. Gradually add milk, stirring with a whisk until smooth. Cook until thick (about 8 minutes); remove from heat. Combine sauce, sausage, pasta, Parmesan, and salt in a large bowl. Spoon mixture into an 11 x 7-inch baking dish coated with cooking spray. Sprinkle with mozzarella. Bake at 400° for 20 minutes or until lightly browned.