Trim the core even with the bottom of the lettuce and cut the lettuce into quarters through the core. If the quarters look too big for a single serving, either cut them in half or trim them to whatever size you feel is a good serving.
Combine the sour cream, yogurt, mayonnaise, garlic, vinegar, and chives in a bowl until well mixed. Season with salt and pepper. Cover and refrigerate until ready to use.
To serve, set the wedge of iceberg in the center of a salad plate, add cucumber, and spoon the dressing over and dash it with a good pinch of chives.