Preheat oven to 400°F.
Spray a 9x13 inch baking dish with non-stick cooking spray. Pour the dry macaroni noodles in the dish with 1 cup cheese and all of the chopped broccoli. Combine and set aside.
1 lb. uncooked elbow macaroni noodles (16 oz.), 4 1/2 cups shredded cheddar cheese*, divided, 2 large heads of broccoli, chopped into small pieces (about 3 cups)
Melt the butter in a medium saucepan over medium heat. Gradually whisk in the flour, whisking constantly for about a minute.
8 Tbsp. unsalted butter, 1/4 cup flour
Pour the milk in gradually, whisking in between increments. Continue until a thin sauce begins to form. Add in the water and whisk again. Bring to a boil, reduce the heat to a simmer.
4 1/2 cups whole milk, 1 cup water
Add in the remaining cheese and seasonings. Simmer for another 10 minutes then remove from heat and pour over pasta and broccoli. (Once again, sauce will be thin).
3/4 tsp. salt, 1/2 tsp. garlic powder, 1/2 tsp. white pepper, 1/4 tsp. ground mustard
Gently stir. Cover with foil and bake for 40 minutes or until edges are bubbling and pasta is tender.
Remove from oven and let rest with foil on for 10 minutes before serve. Stir and enjoy immediately.