Cook the pasta and broccoli together in a large pot of salted water according to package instructions for al dente.
Preheat the oven to 375°F.
In a large heavy skillet, melt the butter over low heat. Add the onion and cook until translucent.
Add the flour and cook for another minute until golden.
Stir in the skim milk and chicken broth, raising the heat to medium-high. Bring to a boil and cook until thickened.
Season with salt and pepper to taste.
Remove from heat and stir in the cheddar cheese until melted.
Add the cooked macaroni and broccoli, mixing well.
Pour the mixture into a baking dish coated with cooking spray.
Top with more cheddar cheese, grated parmesan, and seasoned bread crumbs.
Bake for 15 to 20 minutes, then broil until the top is golden and crispy.
