Prepare the Oven and Pasta: Preheat your oven to 180°C (160°C fan/Gas Mark 4). Bring a large pot of salted water to the boil and cook the pasta for about 2 minutes less than the package instructions. It will finish cooking in the oven. Drain and set aside.

Sauté the Chicken and Vegetables: While the pasta cooks, heat the olive oil in a large, heavy-based pan or Dutch oven over a medium-high heat. Season the chicken pieces with salt and pepper, then add them to the pan. Cook for 5-7 minutes, turning occasionally, until golden brown on all sides. You don't need to cook it through completely. Remove the chicken with a slotted spoon and set it aside.

Cook the Aromatics: Add the chopped onion to the same pan (there should be enough fat left, but add a splash more oil if needed). Sauté for 5 minutes until soft and translucent. Add the sliced mushrooms and cook for another 5-7 minutes until they have released their liquid and started to brown. Add the minced garlic and cook for one minute more until fragrant.

Create the Creamy Sauce Base: Reduce the heat to medium. Add the butter to the pan. Once it has melted, stir in the plain flour to form a paste, known as a roux. Cook for 2 minutes, stirring constantly. This cooks out the raw flour taste.

Build the Sauce: Gradually pour in the chicken stock, whisking continuously to prevent lumps. Once smooth, slowly add the milk, still whisking. I find that using a balloon whisk and adding the liquid in a slow, steady stream is the best way to guarantee a silky-smooth sauce. Bring the sauce to a gentle simmer and cook for 3-4 minutes, stirring, until it has thickened enough to coat the back of a spoon.

Finish the Sauce: Remove the pan from the heat. Stir in the double cream, Dijon mustard, smoked paprika, and about two-thirds of the grated cheddar cheese. Season generously with salt and pepper. Keep stirring until the cheese has completely melted into the sauce.

Assemble the Casserole: Return the cooked chicken to the pan with the sauce, along with the drained pasta and most of the chopped parsley (save a little for garnish). Stir everything together until the chicken and pasta are well-coated.

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 remaining grated cheddar with the panko breadcrumbs. Sprinkle this mixture evenly over the top of the casserole.

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 it rest for 5-10 minutes before serving. This allows the sauce to settle and thicken slightly. Garnish with the remaining fresh parsley.