Step 1 To make the dressing, grate the zest from 1 lemon. Cut both lemons in half. Place the garlic, anchovies and lemon zest in a mortar and pound with a pestle to make a thick paste. Add the egg yolk, a pinch of salt and a squeeze of lemon juice and stir thoroughly to combine. Continuing to stir, pour in 1/2 cup of the oil, drop by drop. The mixture should look smooth and creamy, a sign that you are building a stable emulsion. Continuing to stir, begin adding the oil in a slow stream. The dressing should thicken. Periodically stop pouring in the oil and add a squeeze of lemon. Taste the dressing and add more salt and lemon juice to taste. Add water, a small spoonful at a time, stirring to thin dressing to the consistency of heavy cream. This makes about 2 cups dressing. You may not use all the dressing for the salad; the remainder will keep, covered and refrigerated, for up to 2 days.