Start the pasta water: Fill a large pot with water, add a generous pinch of salt, and bring it to a rolling boil over high heat. You'll know it's ready when the surface is bubbling vigorously and steam rises steadily — this usually takes about 6 minutes on a good hob.

Cook the pasta: Add your spaghetti and cook according to package instructions until al dente, typically 9-10 minutes. While it cooks, reserve about 200ml of the starchy pasta water in a mug before draining — the water should look cloudy and feel slightly slippery between your fingers.

Build the butter base: While the pasta boils, melt the butter in a large frying pan over medium heat. When the butter has fully melted and starts to foam gently, add the sliced garlic. Cook for 1-2 minutes, stirring constantly, until the garlic becomes fragrant and the edges turn a pale golden colour — you'll smell a nutty, sweet aroma when it's ready.

Add the liquid: Pour in the chicken stock and lemon juice, then stir in the lemon zest. Bring the mixture to a gentle simmer — you'll see small bubbles breaking the surface, but it shouldn't be boiling aggressively. Let it reduce for 2 minutes, during which the liquid will thicken slightly and the smell will become distinctly citrusy.

Create the creamy finish: Reduce the heat to low and pour in the double cream, stirring continuously. The sauce will turn a pale, creamy yellow and should coat the back of a spoon. If you're adding shredded chicken, stir it in now and let it warm through for 1 minute.

Combine pasta and sauce: Add the drained pasta directly to the pan with the sauce. Toss everything together using tongs — you'll see the sauce cling to each strand, and the colour will become uniformly glossy. If the sauce seems too thick, add a splash of the reserved pasta water until it reaches a consistency that flows easily but still coats the pasta.

Finish with cheese and parsley: Remove the pan from the heat, sprinkle over the grated Parmesan and chopped parsley, then toss once more. The cheese should melt into the sauce immediately, creating a silky, luscious texture. Taste and adjust salt and pepper as needed.

Serve immediately: Divide the pasta among warm bowls — the sauce will continue to thicken as it sits, so serve right away for the best texture. The final dish should look glossy, with visible flecks of green parsley and yellow zest against the creamy pasta.