1.Set a large pot of water to boil and add the salt. Wash the lemon and zest, avoiding the bitter white part. Squeeze out the juice and set aside.
2.Place the butter, oil and zest in a large non-stick pan. Save some zest to add at the end. Melt the butter and sauté it. Stir well with a wooden spoon and add the lemon juice. Finally add the garlic. Simmer on low heat for about 5 minutes.
3.Take a look at the cooking time of the spaghetti on their package. Halfway through cooking, drain spaghetti and place in skillet with lemon sauce. Spaghetti should be very much al dente, practically still raw, and finish cooking in the pan with the sauce.
4.Add a ladleful of the starch-rich cooking water. Finish cooking the spaghetti in the pan, stirring well. If necessary, add a little more water. Serve hot add zest and parsley and mix well.