Boil the pasta: Bring a large pot of salted water to a rolling boil. Add the fettuccine and cook until just shy of al dente.
Reserve about 1 cup of pasta water, then drain.
Warm the butter and garlic: In a wide skillet over medium heat, melt the butter. If using garlic, add it and cook for 30–45 seconds until fragrant, not browned.
Add the cream: Pour in the heavy cream and bring to a gentle simmer. Stir occasionally and let it reduce slightly for 2–3 minutes until it looks thickened but still loose.
Season lightly: Add a pinch of salt, some black pepper, and the nutmeg if using.
Keep the heat on low to maintain a soft simmer.
Stir in the cheese: Reduce heat to low. Add the grated Parmesan in small handfuls, stirring constantly to melt it smoothly. If it looks too thick, add a splash of reserved pasta water.
Marry the pasta to the sauce: Add the drained pasta to the skillet.
Toss steadily for 1–2 minutes, adding more pasta water as needed. You’re aiming for a glossy sauce that coats each strand.
Taste and adjust: Add more salt or pepper if needed. If the sauce tightens, loosen with another splash of pasta water.
Serve immediately: Plate the pasta while it’s silky.
Finish with extra Parmesan and a sprinkle of parsley if you like.