Coat the bottom of a heavy pot with olive oil and place over medium heat. Add the bell pepper and onion, then sauté until soft and translucent but not browned, about 10 minutes.
Add the garlic and sauté for 1 minute, then add the red pepper paste or dried pepper and paprika and stir well.
Add the tomatoes and cook for 1 minute, then pour in the dry white wine and cook for another minute.
Add the potatoes, along with 2 cups of fish broth and 1 cup water. Bring to a simmer and cook until the potatoes are tender (about 15 minutes). Add a bit more broth or water if necessary to keep the potatoes covered.
Season with salt, pepper, and cayenne (optional), then use the back of a spoon to mash a few of the potatoes to release their starch and thicken the stew.
Add the cubes of tuna and let simmer for 5 minutes. Remove from the heat, cover, and let it stand for 5-10 minutes. Taste a piece of tuna to make sure it's fully cooked.
Serve immediately and enjoy! Although it's not traditional, I sometimes serve this over basmati rice, which makes it last even longer.