Preparation:
Grate the peeled cucumbers and place in a colander, weighted down with a plate for about 15 minutes, so that some of their liquid drains out. Gather up in some muslin and squeeze any remaining liquid out. Place the garlic cloves and a little salt in a mortar. Crush with the pestle until the garlic is pureed.
Put the cucumber, garlic, yogurt, vinegar and pepper into a food processor. Blend until smooth, slowly adding the oil while the blades are running. Cover and refrigerate until ready to use.