Cook the pasta. Bring a large pot of salted water to a rolling boil (use 1 tbsp salt for 6–8 cups water). Add the fettuccine and cook according to package directions until al dente. Reserve 1 cup of pasta water before draining.

Make the lemon butter sauce. While the pasta cooks, melt the butter in a large skillet over medium-low heat. Add the garlic and sauté for 1–2 minutes until fragrant but not browned. Stir in the lemon zest and juice, salt, pepper, and a pinch of red pepper flakes (if using). Let the sauce simmer gently for 2 minutes to blend flavors.

Combine everything. Add the drained pasta to the skillet and toss with tongs to coat the noodles. Sprinkle in Parmesan a little at a time, tossing constantly. If the sauce looks too thick, add reserved pasta water 1–2 tablespoons at a time until it coats the pasta smoothly. Taste and adjust salt or lemon as needed.

Finish and serve. Remove from heat and stir in most of the parsley. Serve immediately with extra Parmesan and parsley on top.