Boil the pasta: Bring a large pot of well-salted water to a rolling boil. Add fettuccine and cook until just shy of al dente. Reserve 1 cup of pasta water before draining.
Melt the butter: In a wide skillet over low heat, melt the butter gently.
Don’t brown it. Add the garlic and cook for about 30 seconds until fragrant.
Add the cream: Pour in the cream and a pinch of salt and pepper. Simmer on low heat for 2–3 minutes, just until slightly thickened.
Keep it gentle; you don’t want it to boil hard.
Whisk in Parmesan: Take the pan off the heat. Sprinkle in the Parmesan in small handfuls, whisking constantly until smooth. If it’s too thick, add a splash of pasta water to loosen.
Toss with pasta: Add the drained pasta to the sauce and toss over low heat for 1–2 minutes.
Add pasta water, a little at a time, until the sauce turns glossy and coats the noodles.
Taste and finish: Adjust salt, add more pepper, and a touch of lemon zest if you like. Top with extra Parmesan and chopped parsley.
Serve immediately: Alfredo is best hot and fresh while the sauce is silky and fluid.