Cook pasta according to package directions, omitting salt and fat; drain.
While pasta cooks, heat a large saucepan over medium-high heat. Add oil; swirl to coat. Remove casing from sausage. Add sausage and onion to pan; cook 6 minutes, stirring to crumble sausage. Add flour, pepper, and salt; cook 1 minute, stirring frequently. Stir in beer; bring to a boil. Cook 3 minutes or until thick and bubbly, stirring constantly. Remove from heat. Add milk and cheeses, stirring until smooth. Stir in pasta. Divide mixture evenly among 4 ceramic gratin dishes coated with cooking spray. Bake at 450° for 10 minutes or until lightly browned.