Preheat oven to 400° and spray a 9 x 13 baking dish with nonstick cooking spray.
In a large pot, bring water to a boil and cook pasta until al dente. Add broccoli to water and boil for an additional 2 minutes. Drain and set aside.
In a large pan, heat oil or butter over medium heat. Add onion and cook for 3-5 minutes or until onions become translucent. Add garlic and mushrooms and cook for and additional 4-5 minutes. Stir frequently.
Sprinkle flour over onions and mushrooms. Stir for approximately 1-2 minutes.
Add chicken broth, stirring constantly until combined, then add milk, mustard, Slap Ya Mama, garlic salt, pepper. Stir until fully combined.
Continue cooking until it reaches a simmer the remove from heat. sprinkle with 1½ cups of shredded cheese. Stir.
Add pasta, broccoli, and cooked chicken, stir to combine the place in prepared baking dish.
Cover with foil and bake for 15 minutes. Remove foil, top with remaining cheese, and bake for an additional 10 minutes or until cheese is melted and bubbly.