Whisk the egg yolks in a bowl, then add the mustard and whisk together. Gradually add about half the oil, very slowly at first, whisking continuously for around 3 to 5 minutes, or until thickened. Once you’ve added about half the oil, whisk in 1 tablespoon of vinegar – this will loosen the mixture slightly and give it a paler colour. Continue to gradually add the remaining oil, whisking continuously. Season with a pinch of salt, a squeeze of lemon juice and a little more vinegar, if needed. Store in a sterilised jar in the fridge for up to one week.