Marmitako is a traditional Basque dish, which I have yet to make myself. I'm told it's delicious and seems very easy to make. I like the one pot idea! I'm unsure of the serving size, it seems like a lot to me for 4 people!
Cut the fish into large, chunky pieces (about 1 1/2 inch squares).
Heat the olive oil in a large stove-top safe casserole dish or pot. (I'd use a nice heavy cast iron pot). Add in the onion, garlic, peppers and tomatoes and fry over a low heat for about ten minutes.
When the onion is looking cooked, add the paprika, cayenne, salt and pepper and cook for another few minutes.
Add in the potatoes then add just enough water to cover the potatoes and veggies. Bring up to a gentle boil.
When the potatoes are nearly done, add in the tuna and the white wine and cook for a further 15 minutes All depending you may need to add a little more liquid if it's drying up, or let some cook off if it's too runny. Garnish with the parsley if using.