Using a tabletop, small mandolin, slice the fennel, celery root and apple into very thin slices. I set mine on the thinnest setting. It's important that they be thin since it's a raw salad. Place in a medium bowl, sprinkle on a little lemon juice to keep from turning brown, and toss. Set aside while you make the dressing.
Mince the clove of garlic as finely as possible, almost to a paste. Place in a small bowl. Add the yogurt, buttermilk, vinegar, lemon juice, and mustard, and whisk together.
Add salt and pepper to taste, and add to the fennel / celery root / apple mixture and toss. Refrigerate for at least 2 hours to allow the flavors to seep into the vegetables.
Just before serving, add the watercress and toss again.Top with pine nuts, pumpkin seeds or hazelnuts.