Slice dark greens from onion and cut some into slivers for garnish. Trim roots and discard. Cut bulb in half lengthwise and slice into slivers (about 1/4 cup).
04In a bowl, whisk together crème fraîche, horseradish, lemon zest and juice, kosher salt and pepper.05Add the onion, herring and chives to the potatoes. Toss the crème fraîche dressing with the potato mixture and combine well. Pour the butter over the mixture and toss gently. Garnish with potato chips and onion greens.