The oil matters here more than in almost any other recipe, because there is nowhere to hide. Avoid grain-based oils, sunflower, rapeseed, vegetable, they sit heavily and bring nothing good. We use grapeseed oil: it comes from the pip, it is light and clean, and it lets the egg and lemon do their work. Olive oil is too strong and will take over completely. Grapeseed is the one.
Everything must be at room temperature, the egg yolk and the mustard especially. Cold ingredients are the most common reason mayo breaks. Take them out of the fridge at least an hour before you start.
For one generous jar: one egg yolk, one teaspoon of Dijon mustard, the juice of half a lemon, a pinch of salt, and around 200ml of grapeseed oil.
Whisk the yolk, mustard, lemon juice and salt together first until combined. Then add the oil, and this is the only rule that matters, drop by drop at the start. Whisk constantly. Once the emulsion takes hold and the mixture begins to thicken, you can add the oil in a thin, steady stream. Keep whisking until it is thick, pale, and holds its shape.
I make mine by hand with a whisk. It takes about ten minutes and is deeply satisfying. If you prefer, a hand electric whisk works just as well, same method, less effort.
If your mayo is not catching, add a small spoon of vinegar and keep whisking. It usually brings it back.
The base is good on its own, but it takes additions beautifully. A clove of raw garlic crushed in makes an aioli. Tarragon, chopped finely, is excellent and worth trying on its own. Smoked paprika and a little cayenne for something with warmth. A teaspoon of wholegrain mustard instead of Dijon for more texture.
Replace the lemon juice with a small splash of soy sauce. Keep the Dijon, it holds the emulsion. The result is darker, deeper, and very good with cold meats and leftovers.
