Boil some water and cook the potatoes, whilst they are cooking drain and crumble the tuna and place into a mixing bowl. Once the potatoes are soft, drain and lightly mash and add to the tuna. Break the eggs, separating the yolk from the white and add the yolks to the tuna mixture along with the parmesan. Season with salt and pepper. Beat the egg whites until stiff and add them to the mixture.
With floured hands, roll the mixture into balls. Place them in the fridge for 1 hour. When the time is up, take them out of the fridge and roll them in a plate of breadcrumbs so that they are evenly covered.
Heat the frying oil. Immerse the tuna croquettes in the oil and leave them to brown for a few minutes, cooking them in small batches. Season with a little salt.
Drain the croquettes on kitchen paper. Decorate with a few coriander leaves. Serve with a green salad and some tomatoes.