1.Cook potato in a large saucepan of salted water for 10 minutes, until tender. Drain and return to saucepan. Cook on medium heat for 1 minute to dry.
2.Remove from heat and mash well. Fold through tuna, parsley, egg and onion. Season to taste.
3.Heat oil in a medium saucepan to 180°C, until a cube of bread sizzles on contact. Spoon tuna mixture, in batches, into oil and deep-fry for 2 minutes, until golden. Drain on paper towel.
4.Drizzle salad leaves with combined oil and vinegar and toss to coat. Serve tuna cakes with lemon wedges and salad.
Use two tablespoons to carefully spoon tuna mixture, shaping as you go, into the oil.