Bring a pot of water to a boil and salt liberally. Add the pasta, give it a stir and cook until al dente according to the package instructions. Reserve 1 cup of the pasta water, then drain the pasta.
Heat a large saute pan over medium heat. Add the pepper and chile flakes if using and toast for 30 seconds. Add the oil and heat until warm. Add the lemon zest and juice and the reserved pasta water and bring to a simmer. Remove from the heat and vigorously whisk in the cheese. Toss in the pasta and parsley if using, then serve .
For more ingredient substitution ideas, see the article below.