Cook the Pasta: Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to the package directions until al dente (still has a slight bite). Just before draining, reserve about 250ml (a mugful) of the starchy pasta water. Drain the pasta and set it aside.

Sauté the Aromatics: While the pasta is cooking, melt the butter in a large, deep frying pan or Dutch oven over a medium heat. Add the chopped onion, carrots, and celery (this trio is known as a mirepoix). Cook for 8-10 minutes, stirring occasionally, until the vegetables have softened and the onion is translucent.

Cook the Chicken and Mushrooms: Add the cubed chicken to the pan and season generously with salt and pepper. Cook for 5-7 minutes, turning the pieces until they are golden brown on all sides. Add the sliced mushrooms and minced garlic and cook for another 3-4 minutes until the mushrooms have released their liquid and are beginning to brown.

Create the Creamy Sauce Base: Sprinkle the plain flour over the contents of the pan. Stir continuously for 1-2 minutes to cook out the raw flour taste – this is your roux. This step is crucial for thickening the sauce.

Build the Sauce: Gradually pour in the chicken stock, about 200ml at a time, stirring constantly to prevent lumps from forming. Once all the stock is incorporated and the mixture is smooth, bring it to a gentle simmer. Let it bubble away for 5 minutes, allowing the sauce to thicken nicely.

Finish and Combine: Reduce the heat to low and stir in the double cream, frozen peas, fresh thyme, and most of the chopped parsley. Let it heat through for a minute, but do not let it boil. Taste the sauce and adjust the seasoning with more salt and pepper if needed.

Bring It All Together: Add the drained pasta to the pan with the sauce. Toss everything together until the pasta is thoroughly coated in the creamy sauce. If the sauce seems a little too thick, add a splash of the reserved pasta water to loosen it to your desired consistency. Serve immediately in warm bowls, garnished with the remaining fresh parsley.