INGREDIENTS
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3 Tbs. unsalted butter
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2 medium leeks (white and light green parts only), trimmed, halved lengthwise, cut crosswise into thin half-moon slices, rinsed thoroughly, and drained
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1 medium yellow onion, thinly sliced
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1 tsp. kosher salt; more to taste
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1-1/2 lb. celery root (about 1 large)
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3/4 cup crème fraîche
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1/4 cup heavy cream; more as needed
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Freshly ground black pepper
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1/4 cup thinly sliced fresh chives