Tonno al Limone (Tuna with Lemon)

Chill a large can of solid pack tuna (not flaked). Open the can, drain the liquid by holding upside down over the sink with the lid pressed to the can. Remove the lid (carefully to avoid getting cut). Put a plate over the open top of the can and invert – the tuna will slide out intact. Squeeze the juice of a lemon over the top. Trim the top with a lemon slice and water cress (or the like). Serve with a variety of crackers.