Mayonnaise is an emulsion which is essentially oil and water held together by egg yolk. Making it teaches control and patience. Adding oil slowly at the start is what allows the mixture to thicken properly. Once you understand this, you understand the principle behind sauces and dressings more broadly. It also costs far less than shop-bought and uses ingredients already in the book.
Place 1 egg yolk, 1 tsp mustard and 1 tbsp vinegar into a bowl.
Wrap a damp tea towel around the base of the bowl to keep it steady while whisking.
Whisk together until combined.
Very slowly add the oil, a few drops at a time, whisking constantly.
Once the mixture starts to thicken, continue adding the remaining oil in a slow steady stream, whisking continuously.
Whisk until thick and glossy.
Season with salt to taste.