Bring a saucepan of lightly salted water to the boil. Add the potatoes and cook for about 15 minutes or until tender.
Meanwhile heat 1 tbsp of the oil in a frying pan and fry the leek gently, without browning for 10 minutes. Halfway through cooking add the peas and stir until they are defrosted and heated through.
Drain the potatoes and mash until smooth.
Stir the Tuna Pate through the mashed potato, then add the cooked leeks and peas, capers and parsley, and mix well together. Season to taste with salt and pepper.
Shape into 4 even sized patties and chill in the fridge for 30 minutes.
Coat the fishcakes first in the flour, then the beaten egg and finally in the breadcrumbs.
Heat a large frying pan with 1cm deep of oil to about 180C ,or until a 2cm cube of bread sizzles and browns in about 45-60 seconds.
Fry the fishcakes for 4-5 minutes on each side until golden. Drain on kitchen paper.
Serve the fishcakes in burger buns with tartare sauce, salad leaves and onion slices.
Tip: Use sweet potato in place of potatoes