Preheat oven to 350 degrees then coat a 2-qt casserole dish with cooking spray. Season chicken breasts lightly with salt and pepper then place in prepared casserole dish. Spritz top of chicken with cooking spray then bake 20–25 minutes or until just cooked through. Remove from oven, cover loosely with foil then let rest. When the chicken is cool enough to handle, shred or cut into bite-sized pieces. Increase oven temperature to 425 degrees.

Put the cauliflower florets in a large microwave-safe bowl. Add ¼ cup water and cover with plastic wrap. Microwave on high for 5 minutes. Uncover the bowl and drain. Set the cauliflower aside.

Heat a large saucepan over medium heat. Melt butter then add sliced mushrooms. Cook for 4 minutes to soften the mushrooms and add diced onion. Cook 3–4 minutes until onion is translucent then add garlic and cook 1 additional minute.

Sprinkle in flour, stirring until dissolved. Cook approximately 1 minute, stirring frequently, then slowly whisk in milk. Bring mixture to a boil then add paprika, dry mustard, chopped parsley, salt and pepper stir to combine. Add cheddar cheese then stir until melted and incorporated. Remove from heat then fold in chicken and cauliflower to fully coat with sauce.

Coat casserole dish with another coat of cooking spray then add the chicken and cauliflower mixture. Combine the Parmesan cheese and breadcrumbs and cover the chicken mixture. Sprinkle top with garlic powder and a pinch more salt then bake uncovered 10–15 minutes until the top is golden brown and the filling is hot and bubbly.

Sprinkle the top with finely chopped parsley and serve.