Prepare the Pasta: Bring a large pot of well-salted water to a rolling boil. Add the fettuccine and cook according to the package directions until al dente (usually about 8-10 minutes). Just before draining, reserve about 250ml (a cup) of the starchy pasta water. Drain the pasta and set aside.

Cook the Chicken: While the pasta is cooking, pat the chicken breasts dry with a paper towel and season them generously on both sides with salt and pepper. Heat the olive oil in a large, heavy-based frying pan over a medium-high heat. Add the chicken breasts and cook for 6-8 minutes per side, or until golden brown and cooked through. Transfer the chicken to a cutting board to rest for 5 minutes before slicing it into 1cm thick strips.

Start the Sauce: Reduce the heat under the frying pan to medium-low. Add the butter and let it melt, scraping up any browned bits from the chicken. Add the minced garlic and cook for about 1 minute until fragrant, being careful not to let it burn.

Build the Cream Sauce: Pour in the double cream and bring it to a gentle simmer. What works best for me is keeping the heat low and steady to prevent the cream from splitting. Let it bubble gently for 2-3 minutes to thicken slightly.

Add the Cheese: Remove the pan from the heat completely. Gradually add the grated Parmesan cheese, whisking continuously until it has fully melted into the cream and the sauce is smooth. This is the most crucial step for a silky texture. Stir in the freshly grated nutmeg and season with a little more salt and pepper to your taste.

Combine Everything: Return the pan to a very low heat. Add the drained fettuccine and the sliced chicken to the sauce. Toss everything together gently until the pasta and chicken are well-coated.

Adjust Consistency: If the sauce seems too thick, add a splash of the reserved pasta water, a tablespoon at a time, until it reaches your desired consistency. The starch in the water helps the sauce emulsify and cling to the pasta. This is the secret to a perfect sauce, as explained in articles on sites like Serious Eats.

Serve Immediately: Stir in the chopped fresh parsley. Serve the chicken fettuccine straight away, garnished with an extra grating of Parmesan cheese and a final crack of black pepper.