Tip the egg yolks and mustard into a bowl, season with salt and pepper and whisk together until completely combined. Whisking constantly, add a small drop of oil and whisk until completely combined, then add another drop and continue a drop at a time until the yolks and oil combine and start to thicken. Once you’re confident the oil and eggs are coming together you can add the oil a bit more at a time, but be patient, as adding the oil too quickly will cause the mayonnaise to split and curdle.
Once all the oil has been whisked into the eggs and you have a thick, spoonable mayonnaise, whisk in the vinegar or lemon juice and season to taste. Will keep in the fridge for two days.
Use as a base to make other saucesMayonnaise is a basic cold sauce which can be used as a base to make lots of other sauces. For tartare sauce, stir finely chopped gherkins and parsley through the mayonnaise. For prawn cocktail (Marie Rose) sauce, stir in 1 tbsp tomato ketchup and a splash of Worcestershire sauce. For aïoli (garlic mayonnaise), make the mayonnaise with olive oil instead of sunflower oil and stir in two crushed garlic cloves at the end.