Preheat oven to 375 Degrees
Cook pasta according to package directions. Add broccoli to the pasta when there's about 3-4 minutes left in the cooking process of the pasta.
While the pasta is cooking: in a large saucepan melt butter, add bread crumbs and cook stirring often until lightly golden brown. Remove from heat and transfer bread crumbs to a small bowl and mix in 1 tbsp grated parmesan cheese.
In a saucepan, whisk together milk and flour until blended, then add onion. Bring to a boil over medium to medium high heat, whisking frequently. reduce heat once boiling and simmer until thickened.
Remove saucepan from heat and whisk in shredded cheese, mustard, salt, and pepper, and remaining parmesan cheese. pour over cooked and drained pasta & broccoli, toss to mix.
Transfer to a baking dish, sprinkle bread crumbs over the top. Bake for 20 - 25 minutes.