Very finely dice the onion. Cook over low heat with a small amount of butter until soft. Soften the remaining butter and stir in onion and remaining ingredients until well combined.
Lay out a piece of foil, plastic wrap or baking paper and place butter along one edge, leaving 2" at each end. Roll up to form a cylinder and twist the ends to seal. Refrigerate until firm. Serve slices on top of hotgrilled meats.