We use a salty chlorophyll-tasting sea succulent called sea banana on this dish, which adds a nice character. It's picked and washed just before service and kept under a damp cloth. Start this recipe a day ahead to marinate the tomatoes.
Note White soy, also called shiro shoyu, is available at specialist Japanese food shops such as Tokyo Mart. Sea bananas are a beach succulent, also known as beach bananas, pigface and karkalla, available at selected greengrocers in season. Xantham gum is a stabilising agent available from health-food shops. You can cold-smoke the fish in a Weber - place smoking chips in a disposable foil tray, heat over a high flame until smoking, then transfer to a Weber with fish and fish roe and close the lid. Sea parsley is occasionally available from Wholefoods House in Waterloo or see Outback Pride for other suppliers.