Put the potatoes in a large pan of cold water. Bring to the boil, then reduce the heat and simmer for 10-15 minutes until tender. Drain and mash, then stir in half the beaten eggs.
Preheat the grill to a medium-high heat. Flake the tuna into the mashed potato and stir through the parsley. Season with black pepper to taste. Form the mixture into 8 evenly sized cakes. Put the breadcrumbs and the remaining egg onto two separate plates. Dip each fish cake into the egg and then roll in the breadcrumbs to coat.
Place the fish cakes onto a baking tray and grill for 15 minutes, turning once. Brush them with the sunflower oil for the last 5 minutes of grilling.
Serve the fish cakes with the coleslaw on the side.