Start by peeling the potatoes and slicing them roughly. Then peel and thinly slice both the onion and the garlic. Pour the oil into a saucepan and let it warm on the hob until it starts to smoke. Then add the potatoes, the onion and the garlic and let these cook for about 5 minutes, turning occasionaly.
Next add the bay leaves, the cayenne pepper, and finally a tsp of sweet paprika. Be careful at this point, you need to get the paprika coated in the oil without burning it - to do this successfully stir non-stop for between 20 to 30 seconds then immediately add the water, until the potato mixture is covered. Remove the cayenne pepper, and let the stew boil for 20 minutes.
Meanwhile, prepare the tomatoes and the fish. To easily peel the tomatoes, scald them (cover for half a minute with boiling water) and then remove the skin. Chop the tomatoes roughly and purée them using a fork. Next, cut the fish into small cubes, carefully removing any remaining bones.
When the 20 minutes are up (this time really depends on the type of potatoes) add the tomato purée and leave the dish simmering on the hob for a further 7 minutes. Then add the tuna, and cook it for just 5 to 7 minutes more - you really have to be careful not to overcook the fish or it won’t be tender. Sprinkle with the chopped parsley and serve in bowls.