Preparation Cook pasta in a 6-quart pot of boiling salted water until al dente. Reserve 1 cup cooking water and drain pasta in a colander. While pasta is boiling, cook garlic and red pepper flakes (to taste) in oil in a 12-inch heavy skillet over moderate heat, stirring, until garlic is golden, about 5 minutes. Stir in zest, then juice, salt, pepper, and 1/2 cup reserved cooking water and bring to a simmer. Toss pasta in sauce with parsley. (Add more cooking water if pasta seems dry.)