Line a large platter with outside lettuce leaves. Place 3 cups of the potato salad in a mound in the center of the platter. Cover with remaining lettuce which has been shredded. Arrange the watercress (if using) on top of the lettuce.
Place the tomato wedges around the outer edge of the salad with a few on the top and place the cucumber in between the tomatoes, making a solid base of the salad. Place the avocado slices around the outside.
Place crumbled feta on top of the salad, with the green pepper slices over all. On the very top, place the sliced beets (if using) with a shrimp on each beet slice and an anchovy fillet on top of the shrimp.
The entire salad is then sprinkled with the vinegar and then with the oil. Sprinkle oregano over all and serve at once. Serve with toasted garlic bread.