Preheat your oven to 350 degrees. Then cook your chicken however you prefer to shred it. For this recipe, I cook it with 1 cup of water in the Instant Pot for 13 minutes. Manually release the pressure, remove the chicken and shred it.
In a pan on medium-high heat, add mushrooms and onions with 1 tablespoon olive oil and saute until the onions are translucent.
In a blender or food processor, combine all of the Cheese Sauce ingredients. Blend until smooth. Set aside a small amount to top the casserole with.
In a large mixing bowl combine, shredded chicken, broccoli, cooked onions and mushrooms, and the cheese sauce. Mix until everything is coated thoroughly with the cheese sauce.
Spray a casserole dish with your favorite non-stick spray and pour your casserole mix into the pan. Top with the cheese sauce that you set aside. Bake uncovered in the oven for 20-25 minutes or until broccoli gets slightly crispy.
Serve and enjoy!