Prep and preheat: Heat your oven to 375°F (190°C). Grease a 9x13-inch casserole dish.

Cook the broccoli: If using fresh, cut into bite-size florets. Blanch in boiling salted water for 2 minutes, then drain and rinse under cold water. Pat dry very well. If using frozen, thaw and squeeze out excess water. Dry broccoli prevents watery casserole.

Warm the aromatics: In a large skillet, melt butter over medium heat. Add garlic and cook 30–60 seconds until fragrant. Stir in onion powder and a pinch of red pepper flakes if using.

Make the cream sauce: Lower heat slightly. Add softened cream cheese and heavy cream, whisking until smooth. Mix in Dijon mustard, 1 cup shredded cheddar, and Parmesan. Stir until melted. If too thick, splash in chicken broth until it’s a pourable, creamy consistency. Season with salt and pepper.

Combine the filling: In a large bowl, toss cooked chicken and broccoli with the cheese sauce until everything is well coated.

Assemble: Spread the mixture evenly in the prepared baking dish. Top with the remaining 1 cup shredded cheddar.

Bake: Bake uncovered for 18–22 minutes, until hot and bubbling around the edges.

Brown the top: For extra color, broil on high for 1–2 minutes. Watch closely so the cheese doesn’t burn.

Rest and serve: Let the casserole sit for 5–10 minutes so it sets up and slices cleanly. Garnish with chopped parsley or chives if you like.