Preheat oven to 375 degrees F. and grease a 9"x13" glass casserole dish with non-stick spray.
Cook pasta according to package directions, add broccoli florets to the water for the final 3 minutes of cook time.
Drain pasta and broccoli when complete. Set aside and return saucepan to the stove top over medium heat.
Add butter to the pan and when it melts, add diced onions. Sauté until translucent, about 5 minutes, then add flour to pan and stir to coat onions. Once flour is mostly dissolved, gradually whisk in milk.
Bring to a boil over medium-high heat, whisking frequently.
Reduce heat to low and simmer until sauce is thick, about 2 minutes.
Remove saucepan from heat and stir in cheddar cheese, mustard, salt and pepper.
Pour over cooked, drained pasta and broccoli. Toss to mix and coat.
Transfer to greased baking dish.
Combine melted butter and breadcrumbs in a small bowl. Feel free to add extra breadcrumbs if the mixture seems too wet. Sprinkle the mixture on top of the broccoli mac and cheese, covering as much as possible.
Bake in oven until bubbly at edges, 20-25 minutes.