I'm putting a copy of this on The Chef's Garden, where more plant people will see it. It's also known as
sour grass, if that's any help. I have a bag of it in my fridge right now, from my local farmers' market. It can be used raw; when cooked, it shrinks down to very little and turns a disgusting shade of green, but it still tastes great.
In the meantime: you could probably substitute baby spinach leaves and a large dose of strained lemon juice, since its most distinctive feature is its tart flavor.