Place your potatoes into a pot, just covered with water. Bring to a boil and cook uncovered for about 10-15 minutes or until fork tender. Promptly drain, transfer to a platter to cool completely.
Tsatziki:
Squeeze out as much of the liquids from the cucumber shreds as you can. I used a paper towel a few times.
In a large bowl, mix together the yogurt, mayo, cucumber, garlic, celery, scallions, lemon juice, olives, dill, parsley and salt and pepper to taste, until smooth. When the potatoes have cooled completely, mix those in.
Garnish with some scallions, dill and freshly cracked black pepper.