Place the egg yolk, mustard and vinegar into a mixing bowl.
Mix the egg, mustard and vinegar together well.
Drizzle a little oil into the egg mixture very slowly, whisking all the time. Add the oil in stages, a drizzle at a time, whisking all the time and in between drizzles. This is a critical stage and to avoid curdling do not rush it.
Keep adding the oil in stages, whisking in all the time. The oil and egg mixture will emulsify (combine to form a mixture with a smooth consistency) and gradually become thicker and thicker. Continue adding the oil until the Mayonnaise is the level of thickness you want.
Add a squeeze of lemon and mix in thoroughly. The colour of the Mayonnaise will become lighter. If the Mayonnaise is no longer thick enough, add a little more oil. Then add salt by degrees, tasting after you mix each small amount of salt in until the Mayonnaise reaches the level of taste you are happy with.
Put the mayonnaise into a container, ideally with a lid and store in the fridge.