Can be prepared in 45 minutes or less but requires additional unattended time.

1 quart buttermilk
1 tablespoon English-style dry mustard
1 teaspoon salt
1 teaspoon sugar
1/2 pound cooked shrimp, cooled, shelled, deveined, and chopped, reserving 2 to 3 whole shrimp, halved lengthwise, for garnish if desired
1 cucumber, peeled, seeded, and chopped fine, plus, if desired, cucumber slices for garnish
2 tablespoons minced fresh chives

In a large bowl whisk together the buttermilk, the mustard, the salt, and the sugar, add the chopped shrimp, the chopped cucumber, and the chives, and stir the soup until it is combined well. Chill the soup, covered, for 3 hours, or until it is very cold. Garnish each serving with 1 shrimp half and a slice of cucumber.