Peel the cucumber and cut in chunks. If it's got a lot of big seeds, scoop some out and discard. Put it in a food processor and chop fine. If it's really moist, or you didn't seed it, then you may want to squeeze off some of the excess juice.
Put the onion in the processor and chop. Add the rest of the ingredients. Process, scrape down the sides, and process again until smooth.
I like it best on soft, white bread. But it's also great on crackers.