In a food processor, mix egg yolk, mustard, pepper, lemon juice, and salt in food processor. Add oil in a small steady stream. Keep chilled.
*RAW EGG WARNING
Food Network Kitchens suggest caution in consuming raw and lightly-cooked eggs due to the slight risk of Salmonella or other food-borne illness. To reduce this risk, we recommend you use only fresh, properly-refrigerated, clean, grade A or AA eggs with intact shells, and avoid contact between the yolks or whites and the shell.
Recipe courtesy of Michael Mina, Aqua