Beat the egg yolks. Add the vinegar, lemon, salt and mustard and beat again for another minute.
In a pouring jug combine the bacon fat and olive oil.
Very gradually add the bacon fat and oil to the eggs. It’s extremely important that the oil is added in a thin, steady stream. I hold the beaker with my left hand and beat with my right. Be wary of over-beating as it can separate your sauce.
The mayo will thicken as it chills in the fridge. It can be kept for up to a week.