Begin by cracking the free range egg and freeing the yolk from the egg white, then place the egg yolk in a mixing jug or bowl. Add the mustard to the yolk along with a large pinch of sea salt, then measure out the oil you need in a separate jug and put the vinegar in another jug. Continue by using a hand blender to whisk together the egg yolk and mustard, after about a minute start to slowly add half of the oil. Take your time and drizzle the oil in while whisking until you have added half, the mixture should now look like mayonnaise already. Continue by slowly adding the vinegar while continuing to whisk and when all of it is in continue to add the rest of the oil until it is all mixed in. Done!