1 extra large egg
125ml good quality white wine vinegar
10-15ml Dijon or mustard of your choice
Optional: I added 10ml fresh thyme leaves to the sauce for potato salad. Chopped chives, parsley, fennel or dill will also work.
Choose a heat resistant mixing bowl that will comfortably fit on the top of a saucepan without the water touching the bowl. Fill the saucepan with boiling water and turn it down to a very gentle simmer.
Place all the ingredients in the mixing bowl and start whisking off the heat to combine the ingredients. Now place the bowl over the steaming water and continue whisking until the foam is pale and is about 3 times the volume. If you lift the whisk the mixture will form a trail on the surface. DO NOT OVERHEAT as you will have a scrambled mass rather than a light foam.
Remove the bowl from the heat and continue whisking for a few minutes until it has cooled down slightly. Pour it over the well drained, cooked vegetables. Serve at room temperature.