Peel the potato and cut into cubes.
Place the potato in a small saucepan and add enough cold water to just cover the potato. Add 1/2 teaspoon salt to the water. Over high heat bring the water to a boil. Reduce the heat to low and cover the saucepan with a lid. Cook the potatoes until tender, about 15 minutes. Test with a skewer to see if they are cooked. It should easily slide into the potato.
Drain the potato when cooked. Return the potato to the empty saucepan for a minute or two to dry further in the residual heat.
Add the potato to a medium size bowl and mash with a fork.
Add the well-drained tuna and finely chopped onion and mix together.
Add the egg and parsley and mix well. The tuna pieces should have broken down to a fairly fine texture.
Season to taste with sea salt and freshly ground black pepper. Cover the bowl with cling wrap and refrigerate for at least 30 minutes to one hour to allow the mixture to firm up.
Shape the mixture into small balls and then flatten into patties, shaping them firmly with your hands. I used an ice cream scoop to make even sized patties.
Place the polenta/cornmeal on a large plate.
Coat the patties evenly in the cornmeal and then move to a separate plate.
Heat the olive oil in a large non-stick frying pan until it is hot but not smoking. Gently add the patties to the pan and cook about 3 minutes each side until the exterior is crisp and golden. Avoid moving the patties around, they are fragile. Leave them to cook and firm up. Turn only once if possible.
Cook the Tuna Patties in batches if necessary. Place the first batch on a plate lined with kitchen paper and keep warm in a low oven.
Serve the Tuna Patties with a wedge of lemon and a delicious salad.Alternatively, they make a great burger. Place them in a burger roll with mayonnaise, tomato slices and shredded lettuce.