Sit a medium sized bowl on a folded tea towel to stop it moving around as you work.
Place the egg yolk, vinegar, mustard, whey, garlic (if using), and salt & pepper into the bowl.
Whisk the ingredients together to combine.
Slowly start to add small amounts of the olive oil, whisking to incorporate the oil between each addition. You can slowly begin to increase the amount of oil added, but ensure each addition is whisked in before more oil is added.
The mixture will start to thicken as the oil is added. You may not need all of the oil to achieve the desired texture, so stop adding oil when you are happy with the consistency.
Taste the mayonnaise and whisk in additional vinegar, salt & pepper if required.
Scoop the mayonnaise into a clean glass jar.
Allow to sit at room temperature until small bubbles can be seen developing in the mayonnaise.
Place the fermented mayonnaise in the fridge and use within one month.