01 - Bring a large pot of salted water to a boil. Add the spaghetti and cook according to package instructions until al dente. Reserve 1 cup of pasta cooking water, then drain the pasta.
02 - While the pasta cooks, heat the olive oil in a large skillet over medium heat. Add the sliced garlic and cook, stirring frequently, until fragrant and lightly golden, about 2 minutes. Avoid burning the garlic.
03 - Stir in the red pepper flakes, sea salt, and black pepper briefly to combine.
04 - Add the drained pasta to the skillet and toss to coat evenly with the garlic oil. If the mixture seems dry, add a splash of the reserved pasta water until desired consistency is achieved.
05 - Remove the skillet from heat. Stir in the grated Parmesan, chopped parsley, and lemon zest if using. Toss well and serve immediately, topped with extra Parmesan and a drizzle of olive oil if desired.