In a large saucepan of salted boiling water cook the spaghetti until it is al dente. While the spaghetti is cooking, in a large heavy skillet cook the garlic in the oil over moderately low heat, stirring, until it is golden, being careful not to let it get too brown, stir the parsley, the tomato purée, and salt and pepper to taste, and cook the sauce, stirring, for 1 minute. Keep the sauce warm. Ladle out and reserve about 1/3 cup of the pasta water, drain the spaghetti, and add it to the sauce with 1/4 cup of the reserved water. Toss the spaghetti mixture over low heat for 1 minute, adding the remaining reserved pasta water if desired, divide it among 2 heated plates, and season it with pepper.