On a really hot summer day, nothing beats cold soup for a refreshing first course or intermezzo. Cucumbers are relatively mild-flavored, and they make an interesting yet neutral base for soups. Here we've added a subtle hint of ginger, lemongrass and Kaffir lime to create a light but sprightly soup. A dollop of crème fraîche makes a pretty garnish and adds a bit of richness.
If you want a stronger flavor statement, add minced jalapeño peppers or fresh mint leaves to the purée. If you use English cucmbers, you don't need to peel or seed them.
Remove the stem ends and top half of the stalks of the lemongrass, and discard. Peel off the outer 2 or 3 layers of the lemongrass until you reach the tender inner core; discard the outer leaves. Finely mince the tender part of the lemongrass and all all but 1 teaspoon to a small saucepan; reserve the 1 teaspoon. Add the chicken stock, ginger and Kaffir lime leaves to the pan and bring to a simmer over low heat. Simmer until the liquid has reduced to 1-1/2 cups, 5 to 10 minutes.
Remove from the heat and let the stock cool and the flavors develop. Strain the mixture through a fine-mesh sieve and discard the solids.
Place the cucumbers, the reserved 1 teaspoon of lemongrass and the scallions in a food processor and purée until smooth. It may be necessary to do this in 2 batches, depending on the size of your food processor.
Add the chicken stock and process again. Add the salt to taste, and refrigerate the soup until it's chilled.
To serve, divide the soup among bowls or cups. Garnish each serving with a dollop of crème fraîche and serve chilled.