Preheat oven to 375 degrees F. Coat a 9 x 9 baking dish with cooking spray.
Set a large pot of salted water to boil. Add pasta and broccoli together; cook according to pasta’s package directions for al dente.
In a large skillet set to medium heat, melt butter. Add onion and cook 1 minute. Whisk in flour and cook another minute. Whisk in milk and chicken broth. Bring to a boil over medium-high heat. Reduce heat to medium and cook sauce about 5 minutes, or until smooth and thick. Season with salt and pepper.
Once sauce has thickened, remove from heat. Add cheddar, stirring until well blended. Add pasta and broccoli; mix well. Pour into prepared baking dish. Top with grated parmesan and breadcrumbs. Lightly coat top with cooking spray.
Bake for 15-20 minutes, then broil for 2-4 minutes to get the breadcrumbs golden.