A cool soup for summer

I like making cold soups in summer, and I have a few favorites that I make each year (yogurt/cucumber, apricot/wine, gazpacho).  But today I happened to have extra buttermilk on hand (left over from making scones), and I noticed a recipe in a new cookbook I'd just purchased that would help me use it up.  This soup is great!  I was dubious about the base of buttermilk, which I've only used before in baking.  I'm a convert!  Actually, it's quite similar to yogurt, but with a bit of a tang.

CHILLED BUTTERMILK-VEGETABLE SOUP

adapted from Vegetarian Meals: Meatless Recipes Everyone Will Love (Good Housekeeping) (NY: Hearst Books, 2006) 

makes about 6 first-course servings

1-2 limes

1 qt. buttermilk (4 cups)

3 medium tomatoes (about 3/4 lb), seeded and cut into 1/4-inch pieces

1 seedless cucumber, chopped in 1/4" pieces (can leave skin on if thin)

1 ripe avocado, cut into 1/4" pieces

3/4 cup chopped cilantro

1/2 t. salt

1/4 t. coarsely ground black pepper

Grate about 1 t. peel from the lime(s) and squeeze to get 2-3 T. juice.

In large bowl, combine peel and juice with the rest of the ingredients.  Cover and refrigerate at least 2 hours.