Place potatoes in a medium saucepan and cover by 1-2 inches with water. Add a generous pinch of salt and bring to a boil. Lower heat to maintain a gentle boil and cook until potatoes until tender, about 15 minutes. Drain potatoes and let cool to the touch before handling.
While potatoes cook, chop onion, celery, green onion, egg and dill.
Chop potatoes in uniform ½-inch cubes.
In a large bowl, combine potatoes, onions, celery, egg and dill. Add yogurt, dijon, lemon juice, salt and pepper. Gently stir everything to combine, adding more yogurt as needed.
Refrigerate, covered, at least 1 hour before serving.
