Gettin' Fancy: Watercress Soup

I had never heard of watercress, or watercress soup for that matter, until I saw it in a vegetarian cookbook I bought a couple years ago. It always looked intriguing, but it kept getting forgotten about. The other day I was at the grocery store though, and happened to see it near the cilantro - and finally decided to try it.
Watercress isn't in season, and this soup isn't really a "winter" soup; but sometimes you just gotta jump on things while they're on your mind and you've got the time. Plus after last night's heavy West African Chicken & Peanut Stew, we needed something light!

1 t olive oil
1/2 sweet yellow onion, chopped
2 cloves garlic
2 c chicken stock (I used the remaining stock from the West African recipe)
1 small potato, peeled & chopped
1 bunch watercress, washed & trimmed
1/2 t cornstarch

Saute onion and garlic in olive oil. Add stock, bring back to boil, and add potato. When potato is softened, add watercress, turn down to simmer, and thoroughly stir in cornstarch. Stir often. After simmering for 10-15 minutes, remove from heat and use an immersion blender to blend thoroughly.

Soup was served alongside chicken and green beans, with a lemon wedge for garnish. The lemon gave it a good tang. All the flavors went well together - plus the soup looks so fancy served in a rocks glass! Watercress soup will definitely be staying on my radar.