It never happened when using this immersion blender method, but just in case, it does. Add 2-3 teaspoons of boiling water and blend again. Place 2-3 tablespoons of the broken mayonnaise in a clean bowl; add an egg yolk, ½ teaspoon mustard, and a few drops of lemon juice. Stir well and only in one direction with a wooden spoon. Once the mayo looks OK again, slowly add the remaining broken mayo, 1 or 2 tablespoons at a time, until the mixture is thick and glossy again.