Add the lemon zest to the skillet and cook over moderate heat until browned, about 1 minute. Add the lemon juice and simmer for 10 seconds. Add the stock and simmer, scraping the bottom of the skillet, until reduced to a rich glaze, about 3 minutes. Swirl in the remaining 1/2 tablespoon of butter. Return the scallops, along with any juices, to the skillet and cook over low heat until heated through.