Preheat and prep. Heat your oven to 400°F (200°C). Grease a 9x13-inch baking dish. Pat the chicken dry and season with 1 teaspoon salt, 1/2 teaspoon pepper, and paprika.

Par-cook the broccoli. Steam or blanch broccoli for 2 minutes until just bright green and slightly tender. Drain well and pat dry. A quick squeeze of lemon over the broccoli is a nice touch.

Sear the chicken. Heat olive oil in a large skillet over medium-high. Cook chicken in two batches for 2–3 minutes per side until lightly golden. It doesn’t need to be fully cooked; it will finish in the oven. Transfer to a bowl.

Make the sauce base. In the same skillet, lower heat to medium. Melt butter, then stir in flour to make a roux. Cook 1–2 minutes, stirring, until it smells nutty. Add garlic and onion powder.

Whisk in liquid. Slowly pour in the milk, whisking to smooth out lumps. Bring to a gentle simmer and cook 2–3 minutes until slightly thickened. Stir in mustard if using. Season with 1/2 teaspoon salt and a pinch of pepper.

Add the cheese. Remove from heat and stir in 1.5 cups shredded cheddar until melted and smooth. Taste and adjust seasoning. The sauce should be well-salted so it flavors the chicken and broccoli.

Assemble the bake. Spread broccoli in the prepared dish. Scatter the seared chicken over the top. Pour the cheese sauce evenly over everything and gently toss to coat, smoothing the surface.

Mix the crust. In a small bowl, combine breadcrumbs, remaining 1/2 cup cheddar, and Parmesan if using. Drizzle with a teaspoon of olive oil or melted butter. Toss until the crumbs look slightly damp and clumpy.

Top and bake. Sprinkle the crumb mixture evenly over the dish. Bake for 18–22 minutes, until the crust is golden and the sauce is bubbling at the edges.

Rest and serve. Let it rest 5–10 minutes so the sauce settles. Sprinkle with parsley and serve with rice, buttered noodles, or a green salad.