You can find Andalusian potato-tuna salads like this offered at any café or bar in Jerez, served at most hours of the day and night. Eduardo Ojeda of Equipo Navazos likes to make the dish as the starter course for family meals at home. He uses tuna belly preserved in olive oil, tins of which are sold in Spain as ventresca de atún claro. He also adds a serious quantity of olive oil, never measuring exactly. Just when you think you've added too much oil, he recommends adding a healthy flourish more. Serve with fino, ideally from Macharnudo vineyard.