Bring a large pot of salted water to a boil and cook the pasta according to package instructions until al dente. Reserve 1 cup of pasta water and then drain the pasta.
In a large skillet over medium heat, melt the butter. Once melted, add the minced garlic and sauté for about 1-2 minutes, until fragrant but not browned.
Stir in the lemon zest and juice, and if using, the red pepper flakes. Cook for an additional minute.
Add the drained pasta to the skillet and toss to coat in the lemon garlic butter. Gradually add reserved pasta water a little at a time until you reach your desired sauce consistency.
Stir in the grated Parmesan cheese until melted and combined. Season with salt and black pepper to taste.
Remove from heat and garnish with freshly chopped parsley before serving.
Notes
Adjust the red pepper flakes according to your spice preference.