Wash the endive salad and cut it into very fine strips, throw away the stalk. Soak the strips in lukewarm water for 10 minutes so that the bitter substances are pulled out a little. Dice the onion very finely. Mix well all other ingredients except for the oil and water. This works best in a blender. When the sugar and salt have dissolved, mix in the oil. Stir well to create a fine emulsion. If the sauce is too intense, you can dilute it a little with water. Put the lettuce in a large bowl, the onions and the sauce as well and mix everything well so that the sauce is lightly coated with the salad.