Place the egg yolks, mustard in a blender or food processor and combine. I used an immersion hand stick blender.

Next, slowly add the oil (very slowly, just drops at a time) and blend with the egg mixture. You will see the mixture start to emulsify (it's like magic, very cool). Do not be tempted to start adding the oil faster, stay at drops until all incorporated.

Stir in the lemon juice.  Add salt to taste (pepper if you choose).

Store in an airtight container in the fridge. Should last about one week. I estimate it made about 1 1/4 cups.