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 the package instructions, but undercook it by about 2 minutes so it’s very al dente. It will finish cooking in the oven. Drain and set aside.

Sauté the Aromatics: While the pasta cooks, heat the olive oil in a large, deep frying pan or Dutch oven over a medium heat. Add the chopped onion and cook for 5-7 minutes, until softened and translucent. Add the minced garlic and sliced mushrooms and cook for another 5 minutes, until the mushrooms have released their water and started to brown.

Cook the Chicken: Add the cubed chicken to the pan. Season generously with salt and pepper. Cook for 6-8 minutes, stirring occasionally, until the chicken is cooked through and lightly golden. Remove the chicken and mushroom mixture from the pan and set it aside.

Make the Roux: In the same pan, melt the butter over a medium heat. Once melted, whisk in the plain flour and cook for 1-2 minutes, stirring constantly. This is called a roux and it will thicken our sauce. This step is crucial for a smooth sauce, and you can learn more about the science behind a perfect roux here.

Create the Creamy Sauce: Gradually pour in the milk, whisking continuously to prevent lumps. I find it's best to add the first 200ml or so very slowly, whisking until smooth before adding the rest. Continue to cook, stirring frequently, for about 5-7 minutes until the sauce thickens enough to coat the back of a spoon.

Add the Cheese: Remove the pan from the heat. Stir in the single cream, Dijon mustard, and smoked paprika. Then, add the 250g of grated Cheddar and 100g of mozzarella. Stir until the cheese is completely melted and the sauce is velvety and smooth. Season with salt and pepper to your taste.

Combine and Assemble: Return the cooked chicken and mushroom mixture to the pan with the cheese sauce. Add the drained pasta and stir everything together until well combined and every piece of pasta and chicken is coated in the sauce.

Bake to Perfection: Pour the mixture into a 9x13 inch (or similar capacity) baking dish and spread it out evenly. In a small bowl, mix the breadcrumbs, the extra 30g of Cheddar, and the chopped parsley. Sprinkle this topping evenly over the casserole.

Final Bake: Bake in the preheated oven for 20-25 minutes, until the topping is golden brown and the sauce is bubbling deliciously around the edges. Let it rest for 5 minutes before serving.