Bring a large pot of very well salted water to a boil. Heat the olive oil in a sauté pan over medium heat. Start cooking the spaghetti. While it cooks, add the garlic to the pan with just enough heat so that it sizzles gently. Add the crushed red pepper flakes. When the outside edges of the garlic are just starting to brown, add 1 cup of the pasta water and a pinch of salt. Keep cooking, shaking the pan to create an emulsion, until the oil and water mixture no longer has that very harsh raw garlic flavor.
When the pasta is al dente, drain it and add it to the pan, along with the chopped parsley, if using, tossing to coat. Serve immediately.