Chilled cucumber and avocado soup

This is a great summery soup - the avocado makes for a lovely cream texture without the addition of cream, while the yogurt adds tartness and bulk.  Adapted from a recipe by Martha Stewart.

Ingredients

1 english (large, 12" or so) cucumber, peeled and diced
1 avocado, pitted, and with the insides scooped out
1/2 t lemon zest
1 T lemon juice
1/4 c fresh cilantro, chopped
a few sprigs of fresh mint, chopped
1 clove garlic, minced or pressed
1 green onion, coarsely chopped
1 c plain yogurt
1 c cold water
salt and fresh pepper to taste

Instructions

1) Puree ingredients in a blender (or with a hand blender) until smooth.
2) Refrigerate for 20 minutes, or until chilled.
3) Serve garnished with a dash of olive oil and a few sprigs of cilantro.