Whisk together the egg yolk, salt, ground mustard, and sugar in a glass bowl. Combine the lemon juice and vinegar in another bowl. Whisk half of the lemon juice mixture into the yolk mixture. On a non-skid pad, whisk briskly, adding the oil a few drops at a time until the liquid seems to thicken and lighten a bit, which means you’ve got an emulsion on your hands. Once you reach that point, you can relax your arm a little and increase the oil flow to a constant, but thin stream. Once half of the oil is in, add the remaining lemon juice mixture.