Take a tall measuring jug or a tall glass container.
Put the egg yolks, Dijon mustard, garlic if using, salt & pepper in the jug.
Mix them together.
Pour in the oil.
Using a hand blender - plunge it all the way to the bottom and then turn it on.
Then very slowly pull it up towards the top of the jug, you will see the mixture turning into mayonnaise as you pull it up.
Add the lemon juice as soon as your mixture has turned into mayonnaise.
Blitz it again with the hand blender, taste & season again if you wish.
Store in a jam jar for up to a week in the fridge.