Broccoli Mac and Cheese

Preheat oven to 375 degrees F. and grease a 9"x13" glass casserole dish with non-stick spray.
Cook 6 ounces uncooked pasta according to package directions, add 10 ounces uncooked 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 1 tablespoon unsalted butter to the pan and when it melts, add diced ½ cup diced onions. Sauté until translucent, about 5 minutes, then add ⅓ cups all purpose flour to pan and stir to coat onions. Once flour is mostly dissolved, gradually whisk in 2 ½ cups 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 1 cup shredded cheddar cheese, 1 teaspoon dijon mustard, 1 ½ teaspoon salt and ½ teaspoon black pepper.
Pour over cooked, drained pasta and broccoli. Toss to mix and coat.
Transfer to greased baking dish.
Melt ¼ cup unsalted butter and mix with 1 cup Italian style 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.