I primarily find myself using buttermilk in baking recipes, where it's tangy flavor adds unmatched depth to biscuits or pancakes.  However, I always enjoy using the leftover portion from the quart in simple recipes that play-up buttermilk's unique taste.  This soup takes minutes to make and is wonderfully refreshing.

Radish and Buttermilk Soup
Adapted from Gourmet
Serves 2 - 3

Ingredients

1.25 C radishes, quartered
2 C seedless cucumber, sliced (and peeled if you would like)
2 C chilled buttermilk, well-shaken
1 tsp salt
1 tsp rice vinegar
.5 tsp sugar

Method

Puree all ingredients in a food processor until smooth.  Serve chilled.