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. Increase oven temperature to 425 degrees.

Fill a large saucepan halfway with water then bring to a boil over high heat. Add cauliflower and a few pinches of salt. Cook cauliflower approximately 3 minutes until fork tender but still slightly crunchy. Drain and set aside.

Return pot to stove then reduce heat to medium. Melt butter then 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, salt and pepper and stir to combine. Add Parmesan 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 cooking spray then add the chicken and cauliflower mixture. Sprinkle top with cheddar cheese and breadcrumbs then drizzle with melted butter. Sprinkle top with garlic salt then bake uncovered 10–15 minutes until the top is golden brown and the filling is hot and bubbly.