01 - Bring a large pot of salted water to a boil. Add spaghetti and cook until al dente according to package instructions. Reserve 1/2 cup pasta water, then drain pasta.
02 - While pasta cooks, heat olive oil in a large skillet over medium-low heat. Add sliced garlic and sauté, stirring frequently, until fragrant and lightly golden, about 2 minutes. Avoid burning the garlic.
03 - Add crushed red pepper flakes if using, then immediately transfer drained pasta to the skillet. Toss thoroughly to coat the pasta in the garlic-infused oil.
04 - Gradually add reserved pasta water, a little at a time, stirring until pasta is silky and lightly coated with sauce.
05 - Stir in chopped parsley, sea salt, black pepper, and grated Parmesan cheese. Toss to combine evenly. Add lemon zest for a bright finish if desired.
06 - Serve immediately, topped with additional Parmesan cheese.