Preheat and Prepare Rice: First, preheat your oven to 180°C (160°C fan). Cook the rice according to the package instructions. Once cooked, drain it and set it aside.

Cook the Chicken: While the rice is cooking, heat the olive oil in a large frying pan over a medium-high heat. Add the chicken pieces, season with salt and pepper, and cook for 5-7 minutes, until golden brown and cooked through. Remove the chicken from the pan and set it aside.

Blanch the Broccoli: Bring a small pot of water to a boil. Add the broccoli florets and cook for just 2-3 minutes until they turn bright green and are slightly tender. Drain immediately and rinse with cold water to stop the cooking process. This keeps them from getting mushy in the casserole.

Start the Cheese Sauce: In a large saucepan, melt the butter over a medium heat. Add the chopped onion and cook for 4-5 minutes until softened. Add the minced garlic and cook for another minute until fragrant. Stir in the plain flour to create a paste (this is your roux) and cook for 1-2 minutes, stirring constantly. You can learn more about making a perfect roux sauce to build your confidence.

Finish the Sauce: Gradually whisk in the warm milk, a little at a time, ensuring each addition is fully incorporated before adding more. This prevents lumps. Once all the milk is added, whisk in the chicken stock. Bring the sauce to a gentle simmer and cook for 3-4 minutes, stirring, until it has thickened.

Add the Cheese: Remove the saucepan from the heat. Add about 200g of the grated cheddar, the Dijon mustard, and the nutmeg. Stir until the cheese is completely melted and the sauce is smooth. Season generously with salt and pepper to your taste.

Combine Everything: In a large mixing bowl, combine the cooked rice, cooked chicken, and blanched broccoli. Pour over the cheese sauce and gently stir until everything is evenly coated. What works best for me is using a large spatula to fold everything together to avoid breaking up the broccoli.

Bake the Casserole: Transfer the mixture to a 9x13 inch baking dish and spread it out evenly. Sprinkle the remaining 50g of cheddar cheese over the top. If using, mix the panko breadcrumbs with the melted butter and scatter them over the cheese.

Bake and Rest: Bake in the preheated oven for 20-25 minutes, or until the top is golden brown and the sauce is bubbling around the edges. Let the casserole rest for 5-10 minutes before serving. This allows it to set slightly, making it easier to serve.