Quickly Bring The Egg to room temperature by setting it in a bowl of hot water for 2-or-3 minutes, or until the egg no longer feels chilled when held in your hand. Then separate out the yolk. Combine the egg yolk, salt, mustard and lemon juice or vinegar in a bowl; set the bowl on a folded towel to keep it from moving around. Briskly whisk the ingredients together until they are thoroughly blended; then begin adding the oil, drop by drop at first, then in gradually increasing amounts. When the oil is completely incorporated, taste the mayonnaise and add more salt or lemon juice or vinegar if desired. A very thick mayonnaise can be thinned by stirring in a spoonful or two of hot water until you get the consistency you want.