Serve this simple spread with crostini or plain, crisp crackers. And please, use the best tuna in olive oil you can find; tuna packed in water or vegetable oil just won’t cut it for this recipe.
In a small saucepan, cover potato with cold water by at least 1 inch. Add a good pinch of salt and bring to a boil over high heat. Reduce heat to medium, cover pan partway, and cook until potato is tender, about 8 minutes. Drain and return potato to pan. Dry the potato, shaking pan, over medium heat, until it is floury and has left a film on the bottom of the pan. Rice potato into a bowl and set aside to cool. In a food processor, combine tuna (with its oil), parsley, capers, anchovies, oil, and vinegar. Process, scraping down sides a few times, until the mixture is a very smooth paste. Season with cayenne. Fold tuna paste into potato thoroughly, until there are no lumps of potato. On a small platter, form pate into the shape of a fish. Chill at least 1 hour. Right before serving, garnish “fish” with cucumber slices as scales and a caper as an eye.