That's it. Refrigerate until needed, but remember that with a large butter content you'll want it to warm somewhat before serving or it won't be very...
Step 2: Chop capers
Since the capers have the strongest flavor, they need to be the most finely chopped. On the other hand, they start out smallest too, so you can chop them at the same time as the onion.
Throw appropriate amounts of capers and onions into your food processor and chop them up some. You don't need to get them TOO fine at this point; they'll get chopped some more as we add the other ingredients.