Place a bowl on top of a cloth on the working space (so that the bowl does not shift while beating). Add egg yolk, mustard and season with salt and pepper. Beat this with a whisk. Slowly add the oil while still beating. When the mayonnaise is starting to bind, add oil in larger quantities. Continue beating. When the oil is fully mixed through the egg yolk, continue beating for another 30 seconds until the mayonnaise is shiny. Add the vinegar or lemon juice to taste. If needed, add a bit more vinegar and season with salt and pepper.