Boil a large pot of water, add a generous pinch of salt, and cook pasta just to al dente, according to package directions. Before draining, scoop out 1 cup of the cooking water into a liquid measuring cup. Set this aside, then drain the rest of the water from the pasta.
12 ounces spaghetti, bucatini, or another long, thin pasta
While the pasta is bubbling, set a large skillet over medium-low heat. Add the olive oil. As soon as the oil starts to shimmer, add the slices of garlic. Cook for about 2 minutes, stirring frequently, until the garlic is fragrant and golden. It should be toasted but not at all burnt.
1/3 cup extra-virgin olive oil, 10-12 cloves garlic
Add red pepper flakes, if using, and toast for just 15-20 seconds. (See note.)
1/8 teaspoon red pepper flakes
Quickly pour the full 1 cup of reserved pasta water into the skillet. Increase heat to medium-high and bring the mixture to a rapid simmer. Let it bubble rapidly for 4-5 minutes, until visibly reduced. (Don’t rush this step—it’s important to emulsify the sauce and make it the right consistency to cling to the pasta.)
Add pasta and toss very well to combine.
Stir in the Parmesan and minced parsley, if using, then remove the pan from the heat. Toss for a minute or so to let the cheese melt, then sample and season with salt and black or red pepper, to taste. Enjoy!
1/2 cup grated Parmesan cheese, 2-3 Tablespoons minced fresh parsley, kosher salt and black pepper