A whole range of flavor possibilities can be achieved by varying the herbs in this recipe, and adding the yogurt both frees up the consistency and gives a welcome tartness.
Place egg yolk, herbs, vinegar, mustard, and 2 tablespoons of the oil in a food processor or blender. Mix until smooth. Slowly dribble in more oil as needed to make a smooth mayonnaise. To make the consistency of salad dressing, add yogurt, season with pepper, and mix well.