In a large pot of boiling salted water, cook the spaghetti or linguine according to package instructions until al dente. Reserve 1 cup of pasta water, then drain the pasta and set aside.
In the same pot over medium heat, melt the butter. Once melted, add the minced garlic and red pepper flakes. Sauté for about 2-3 minutes, or until the garlic is fragrant and lightly golden but not browned.
Add the drained pasta to the pot with the garlic butter. Toss to coat the pasta evenly in the delicious butter sauce.
Gradually sprinkle in the grated Parmesan cheese, and pour in a bit of the reserved pasta water (starting with 1/4 cup). Toss continuously until the cheese has melted and coated the pasta smoothly. Add more pasta water as needed to reach your desired sauce consistency.
Stir in the chopped parsley and lemon zest, and season with salt and pepper to taste. Toss everything together one last time.
Plate the pasta and finish with extra grated Parmesan cheese on top. Garnish with additional parsley or a sprinkle of red pepper flakes if desired.
Notes
Adjust red pepper flakes to taste for spice preference.