Bring a large pot of water to a boil. Once you have a rolling boil, toss a generous amount of salt into the water. Add the pasta and, going by the instructions on the packaging, cook it to al dente.
In a large pot or Dutch oven, heat the oil over medium-low heat. When it's hot, add the garlic and chili flakes, giving them a good stir to prevent burning. Stir them often to keep them moving. (Avoid browning thr garlic.)
When the pasta is ready, reserve 1 cup of the starchy pasta water; drain the rest.
Transfer the pasta to the garlic and chili oil and, using tongs, toss the pasta thoroughly until it's all covered. Add the pasta water bit by bit to create a sauce out of the oil. This will also help the sauce stick to the noodles.
Do a taste test with the pasta and, If it seems bland, add a few pinches of salt until you get the flavor going. This is why salting the pasta water enough is so important.
Stir in the lemon juice, lemon zest, fresh parsley, and a few cracks of black pepper if you wish. Now would also be the time to add parmesan cheese if you're using it.
Toss well to coat and serve with extra chili flakes for those who like a little extra heat.