Place the egg yolk and salt in a food processor. Pulse to combine. Add the lemon juice and mustard; blend well. With the motor running, add the oil, drop by drop. This will take a few minutes. Don't rush it or the mayonnaise may "break," meaning the oil will separate from the egg. (Note: If your food processor has a small hole in the feed-tube pusher, pour the oil in there and let it drip through.) Once you've added the oil, sample the mayo and add more salt or lemon juice to taste. Cover tightly and refrigerate for up to 5 days. Stir before spreading.