How to Make It
Combine vinegar and 1/2 cup water in a small saucepan; bring to a boil. Add onion; return just to a simmer, then remove from heat. Let stand until cooled to room temperature.
Meanwhile, place potatoes in a large saucepan or Dutch oven; cover with cool water. Bring to a boil; reduce heat to medium-low, and cook until potatoes are just tender when pierced with a knife, about 15 minutes. Drain and place potatoes in a large bowl of ice water; cool 5 minutes.
Combine crème fraîche, mayonnaise, buttermilk, chives, dill, parsley, salt, and pepper in a large bowl. Cut potatoes into quarters. Add potatoes to bowl; toss gently to coat with dressing. Drain onion; reserve about 3 tablespoons onion slices. Chop remaining onion, and fold into salad. Spoon salad into a large bowl; top with reserved onion slices.