Culinary use for the green parts of lemongrass?
The outer green leaves are a bit tough, and not a lot of flavor, but the upper section of the inner green stalk you can use for infusing in soups, curries, broths, etc. 

Rough it up a bit, so to speak, bruise it and make a few slits with a paring knife, then let it sit in your broth or curry until you've zapped the last bit of flavor from it. Just remember to remove it before serving to your guests since it isn't exactly edible on its own.