Fry the onion and garlic together in the oil till soft. Drain the oil or brine from the tuna and discard. Place the tuna in a bowl along with the cooked onion, garlic, potato and the fresh herbs and mix together using a fork. Season well with salt and pepper and add in the capers and breadcrumbs. Mix together well before placing the tuna mixture in the fridge for at least 30 minutes. Remove the chilled tuna mixture from the fridge and mould the mixture into small fishcakes using your hands. Roll the fishcakes in the flour, covering all over. Next, heat a little 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 either hot or cold.