Mayonnaise is an emulsion, consisting of egg yolks and oil. Egg yolks and oil would not naturally come together, but when the oil is slowly whisked into the egg yolks, the two liquids form a stable emulsion that won't separate. Mayonnaise is commonly used on sandwiches, hamburgers, salads, seafood, and French fries. It also forms the base for various dressing such as dijonnaise, remoulade, thousand island, rouille etc.. When making mayo, use the best eggs!..
In a medium bowl, whisk together the egg yolk, mustard, salt and pepper. Add the oil in a thin stream, and whisk until homogenized. Then add lemon juice. Refrigerate for up to 2 days.
Note that mayo can be done using an immersion blender. In this case and for practical purpose, the amount of ingredients should be doubled and use a narrow plastic or glass container to ease the procedure.