Prep the oven and dish: Preheat oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.
Par-cook the broccoli: Steam or microwave until crisp-tender, about 3–4 minutes for fresh or 4–5 for frozen. Drain well to avoid a watery bake.
Make the sauce base: In a medium saucepan over medium heat, melt butter.
Add garlic and cook 30–60 seconds until fragrant.
Build the cream sauce: Whisk in heavy cream and cream cheese. Stir until smooth, then add onion powder, smoked paprika, ground mustard, salt, and pepper.
Add the cheese: Stir in 1 cup of the shredded cheese and the Parmesan. Mix until melted and silky.
Combine: In the baking dish, spread the chicken and broccoli.
Pour the cheese sauce over and toss gently to coat everything evenly.
Top and bake: Sprinkle the remaining ½ cup shredded cheese over the top. Bake for 18–22 minutes, until bubbling around the edges.
Broil for color (optional): Broil 1–2 minutes to brown the top. Watch closely.
Rest and serve: Let it sit for 5–10 minutes to set.
Garnish with parsley and serve warm.