Place the egg yolks, garlic, mustard and salt in a small blender/food processor and with the machine running, drizzle the olive oil through the feeding tube. Or else, do it all by hand, as I do and add the oil a little at a time, whisk to blend before each new addition of oil.
When it's all blended in, add the lemon juice and mix well.
Taste and season with more salt if needed and pepper if you like.