Make sure that all ingredients are at room temperature. Start by mashing the hard boiled egg yolk into a bawl. Add the 4 raw egg yolks, salt, white pepper (optional), mustard and mix them together. Next, start using the mixer at low speed and incorporate the oil in a thin stream, bit by bit until soft waves starts forming. After that, you can speed up the process with no stress, adding the oil in a continuous stream until you rich the desired consistency.
Check the seasoning (salt and pepper) and add the lemon juice. That will lighten up the texture and color of the mayonnaise. Finish up by splashing 4-5 tbsp of cold water to lighten the mayo even more and making it more silky and smooth. This is optional, but highly recommended.
Use it as it is, or flavor it according to your liking.
Notes
Make sure you only use fresh raw eggs. This mayo can last up to a week in the fridge.