Preheat and Cook Pasta: First, preheat your oven to 180°C (160°C Fan). Bring a large pot of salted water to a boil and cook the penne according to package directions, but for 2 minutes less than the recommended time (al dente). This is important as it will continue cooking in the oven. Drain and set aside.

Cook the Chicken: While the pasta is cooking, heat the olive oil in a large frying pan or skillet over medium-high heat. Season the chicken pieces with salt and pepper. Add the chicken to the pan in a single layer and cook for 5-7 minutes, turning occasionally, until golden brown and cooked through. Make sure the chicken is cooked to a safe temperature, as advised by the FSA. Remove the chicken from the pan and set aside.

Start the Alfredo Sauce: In the same pan, reduce the heat to medium and add the 100g of unsalted butter. Once melted, add the minced garlic and cook for about 1 minute until fragrant, being careful not to let it burn.

Build the Creamy Sauce: Pour in the double cream and bring it to a gentle simmer. Let it bubble gently for 3-4 minutes, stirring occasionally, until it has thickened slightly. I find that a silicone whisk works best for keeping the sauce smooth.

Add Cheese and Seasoning: Remove the pan from the heat. This is a key step to prevent the cheese from splitting. Gradually stir in the 150g of grated Parmesan cheese, the Italian herbs (if using), and the pinch of nutmeg until the cheese has completely melted and the sauce is velvety smooth. Taste and adjust seasoning with more salt and pepper if needed.

Combine Everything: In a large mixing bowl, combine the cooked pasta, the cooked chicken pieces, and the Alfredo sauce. Gently stir until everything is evenly coated.

Assemble the Casserole: Pour the mixture into a 9x13 inch (or equivalent) baking dish and spread it out evenly.

Add the Topping: Sprinkle the 150g of grated mozzarella and the remaining 50g of Parmesan cheese evenly over the top.

Bake to Perfection: Place the dish in the preheated oven and bake for 20-25 minutes, or until the top is golden brown and the sauce is bubbling around the edges.

Rest and Serve: Remove from the oven and let the casserole rest for 5-10 minutes before serving. This allows the sauce to set up slightly. Garnish with fresh chopped parsley and serve warm.