Set a large pot of salted water over high heat and bring to a boil. Add the fettuccine and cook according to package instructions.
Meanwhile, place the butter and garlic in a large skillet over medium heat. Saute for 2-3 minutes, then add the cream and bring to a simmer. Stir in the shredded cheese, lemon zest and nutmeg. Stir until the alfredo sauce is smooth. Then check the pasta. Once the pasta is "al dente" (cooked, but firm) scoop it out of the water (reserving at least 2 cups of water) and into the alfredo sauce. Stir and cook the pasta and sauce together for 1-3 minutes. Salt and pepper if needed, and add a pinch of crushed red pepper if desired.
If the alfredo sauce starts to tighten too much before you are ready to serve the fettuccine alfredo, stir in some of the pasta water to loosen the sauce. Serve warm with a sprinkle of chopped parsley over the top!