Bring a large pot of salted water to a boil. Add the spaghetti and cook according to package directions until al dente. Reserve about 1/2 cup of pasta water before draining.

While the pasta is cooking, melt the butter in a large skillet over medium heat.

Add the minced garlic and red pepper flakes (if using) to the melted butter and cook for about 1 minute, or until fragrant. Be careful not to burn the garlic.

Stir in the lemon juice and Parmesan cheese. Cook for 1-2 minutes, or until the sauce is slightly thickened.

Add the drained pasta to the skillet with the lemon butter sauce. Toss to coat. If the sauce is too thick, add a little of the reserved pasta water until you reach your desired consistency.

Season with salt and pepper to taste. Stir in the fresh parsley (if using).

Serve immediately, topped with extra Parmesan cheese if desired.