01 - Bring a large pot of salted water to a boil. Cook fettuccine according to package instructions until al dente. Reserve 1/2 cup pasta water, then drain.
02 - While the pasta cooks, melt the butter in a large skillet over medium-low heat. Add the heavy cream and bring to a gentle simmer, stirring frequently, for 2-3 minutes.
03 - Reduce the heat to low. Gradually whisk in the grated Parmesan until the sauce is smooth and creamy. Season with salt, black pepper, and nutmeg if using.
04 - Add the drained fettuccine to the sauce, tossing to coat. If the sauce is too thick, add some reserved pasta water, a tablespoon at a time, until desired consistency is reached.
05 - Remove from heat. Plate immediately, garnishing with chopped parsley and extra Parmesan.