Fry the onion and garlic together in some oil till soft, and leave to cool. Drain the oil or brine from the tuna and discard. Place the tuna in a bowl, along with the beaten egg, the cooked onion and garlic and the fresh herbs. Mash the cooked potato, and add to the bowl. Mix all the ingredients together, adding enough of the breadcrumbs to bind the mixture together. Season well with salt and pepper and place the tuna mixture in the fridge for one hour. Meanwhile, place the flour in a bowl and season well. Remove the chilled tuna mixture from the fridge on mould the mixture into small fishcakes using your hands. Roll the fishcakes in the seasoned flour, covering all over. Next, heat the oil in a large frying pan, and shallow fry the fishcakes for roughly 10 minutes, turning around once. When the fishcakes are cooked, they should be golden brown in colour. Serve the tuna fishcakes with mint either hot or cold.