Preparation

• In a 4-quart or larger heavy-based pot, melt the butter over medium-low heat. Add the leeks, onion, and a generous pinch of salt and cook, stirring occasionally, until very soft and lightly golden but not brown, 15 to 20 minutes. Reduce the heat to low if you see signs of browning.
• Meanwhile, peel the celery root with a sharp knife (expect to slice quite a bit off the exterior as you trim). Halve the peeled celery root lengthwise and cut each half into 1-inch-thick wedges. Cut each wedge crosswise into 1/4-inch slices. You should have about 5 cups.
• 将芹菜根，1茶匙盐和1/2杯水加入韭菜。盖上并煮熟，直到芹菜根变软10到15分钟。（偶尔检查；如果所有水都煮熟并且蔬菜开始变成褐色，请再加1/2杯水。）加入4-1/2杯水，煮熟，然后继续煮20分钟。让稍微冷却。
• Purée the soup (with a hand blender, or in small batches in a stand blender) to a very smooth, creamy consistency. Let cool completely and then store in the refrigerator at least overnight or for up to two days.
• 食用前约一个小时，将奶油奶油放在一个小碗中，搅拌到足够的浓奶油中，使混合物达到酸奶的稠度。将奶油混合物放在室温下，直到准备好汤。（如果奶油太冷，它会冷却汤。）
• Reheat the soup. (If it’s too thick, gradually thin it with as much as 1 cup water.) Taste and add more salt as needed. Ladle the soup into small espresso cups or shot glasses. Top each portion with a small spoonful of crème fraîche (it should float on top of the soup). Finish each cup with a pinch of black pepper and a sprinkle of chives.