Preheat the oven and prep the dish: Heat your oven to 375°F (190°C). Grease a 9x13-inch baking dish.

Cook the pasta: Boil the macaroni in well-salted water until just shy of al dente, about 1 minute less than the package suggests. Drain and set aside.

Blanch the broccoli: In the last 2 minutes of the pasta’s cook time, add the broccoli to the pot. Drain both together. This keeps the broccoli bright and crisp-tender.

Make the roux: In a large pot, melt the butter over medium heat. Stir in the flour and cook 1–2 minutes, whisking, until it smells nutty but not browned.

Build the sauce: Slowly whisk in warm milk and chicken broth. Keep whisking to avoid lumps. Simmer 4–5 minutes until thick enough to coat the back of a spoon.

Season the sauce: Stir in garlic (or sauté fresh garlic in a splash of olive oil before adding), onion powder, mustard, paprika, 1 teaspoon salt, and 1/2 teaspoon black pepper. Taste and adjust.

Add the cheeses: Off the heat, stir in cheddar, Monterey Jack, and half of the Parmesan until smooth. The sauce should be glossy and pourable.

Combine everything: Fold the drained pasta, broccoli, and chicken into the sauce. If it seems too thick, splash in a bit more milk or broth. You want creamy, not soupy.

Assemble: Pour into the prepared baking dish. Sprinkle with the remaining Parmesan. If using, toss panko with melted butter and scatter over the top.

Bake: Bake 20–25 minutes, until bubbling around the edges and the top is lightly golden. For extra color, broil 1–2 minutes at the end, watching closely.

Rest and serve: Let it rest 5–10 minutes so the sauce settles. Scoop and serve warm.