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Pour the soy milk into a jug along with the vinegar and mustard, if using. Use a hand blender to whizz for 30 secs until frothy and voluminous.
Begin to slowly pour in the oil in a steady stream, whizzing the mixture all the time and moving the hand blender up and down so everything is properly combined. Pour into a jar. Will keep in the fridge for up to 2 weeks. Stir before each use.