In a large pot of salted water, cook the pasta, according to package directions for al dente. Spray a baking dish with cooking spray, and set aside. Preheat oven to 375 F. In a large. heavy skillet, melt the butter, then add the onion and olive oil and cook over low heat for 2 minutes. Add the flour and cook 1 minute or until flour is golden and well-combined.
Whisk in milk and broth, raise heat to medium-high until it comes to a boil, then cook an additional 5 minutes or until smooth and thick. Season with salt and pepper if desired. Remove from heat, add shredded cheese, and stir until melted. Stir in the cooked pasta then pour into the prepared baking dish. Top with parmesan. Spray a little more cooking spray on top. Bake 15-20 minutes, then broil 3-5 minutes until the grated Parmesan cheese crumbs are golden.
Let cool, then eat your heart out baby!