Place the potatoes in a large pot of cold water. Add 1 tablespoon salt, the bay leaf and the thyme and bring to a boil. Lower heat to a simmer and cook until potatoes are tender, about 25 minutes. Drain and set aside to cool.
Combine mayonnaise, sour cream, buttermilk, egg, dill, mustard, celery, scallions and red onion in a large bowl and mix well. Season with remaining salt.
When the potatoes are completely cool, gently fold them into mayonnaise mixture until evenly coated. Cover and refrigerate until ready to serve.