1 t mustard
pinch of salt & pepper
Everybody always says Dijon mustard, but I don't like Dijon so I used some honey-mustard instead.
Then whisk in 150 ml peanut/arachis oil. Starting with a few drops, whisk vigorously until it not only blends, but also thickens. Then add a little bit more oil, whisk, add more until after a while you're confident enought to add the rest of the oil in a little, steady stream.
When all the oil is blended in, add a teaspoon of ricewine vinegar or some lemonjuice.
And today I noticed it is really, really vital all ingredients are at roomtemperature. My kitchen was too cold, so my first batch failed. :( Luckily when I started again, with a new eggyolk I could wisk in the "failed/runny mayo" and still come out with an acceptable mayonnaise.
Never, really never use olive oil. I've tried that once and it was really disgusting.