Cook pasta and broccoli together in a large pot of salted water, according to package directions for al dente (or slightly undercook 2 minutes). Spray a baking dish with cooking spray.
Preheat oven to 375°F.
In a large, heavy skillet, melt butter. Add onion and cook over low heat about 2 minutes, add flour and cook another minute, or until the flour is golden and well combined. Add milk and chicken broth and whisk, raising heat to medium-high until it comes to a boil; cook about 5 minutes or until the sauce becomes smooth and thick. Season with salt and pepper.
Once the sauce is thick, remove from heat, add cheese and mix well until cheese is melted. Adjust salt and pepper to taste, add cooked macaroni and broccoli and mix well. Pour into prepared baking dish. Top with grated cheese and breadcrumbs. Spray a little more cooking spray on top.
Bake for 15-20 minutes, then broil for a few minutes to get the breadcrumbs golden.