How to make mayonnaise.
2 egg yolks, 500ml oil, 3 tablespoons lemon juice, 1 tsp mustard, salt and pepper
Using a hand whisk beat the 2 egg yolks with the mustard, slowly drip the oil teaspoon by teaspoon, whisking every drop in before adding more, as it thickens you can add more and more oil. Do it very carefully, it will get thicker and thicker. It helps if someone else can pour the oil in while you whisk. Once most of the oil is in whisk in lemon juice a tablespoon at at time. It changes in an amazing way. Add salt and pepper, taste adjust lemon juice and seasoning.
To this you can add parsley or other herbs, a crushed clove of garlic, you can use different vinegars instead of lemon juice, different oils for different flavours. I made this one with olive oil.
I did read that mayonnaise with olive oil didn’t keep in the fridge because of the way olive oil reacts in the fridge. However I kept the last one in the fridge for a couple of days it became even better as the flavours mingled and deepened.
If your mayonnaise splits the way to recover it is to simply re-do it using another egg yolk. Beat egg yolk and add the separated oil and egg very slowly drip by drip.