Heat the olive oil in a large pot or Dutch oven over medium-high heat.
Add the chopped onion and sauté for a few minutes until it becomes translucent.
Add the minced garlic and cook for another minute until fragrant.
Stir in the sliced red and green bell peppers and cook for about 5 minutes until they start to soften.
Add the chopped tomatoes and cook for another 5 minutes, allowing them to break down and release their juices.
Stir in the potatoes and cook for a few minutes, mixing them well with the other ingredients.
Pour in the white wine and let it simmer for a couple of minutes to allow the alcohol to evaporate.
Add the fish or vegetable broth, bay leaf, paprika, and season with salt and pepper to taste. If you like your Marmitako spicy, you can also add some red pepper flakes at this stage.
Bring the mixture to a boil, then reduce the heat to a simmer and cook for about 15-20 minutes or until the potatoes are tender.
Gently add the tuna chunks to the pot, making sure they are submerged in the broth. Simmer for an additional 5-7 minutes or until the tuna is just cooked through. Be careful not to overcook the tuna as it can become tough.
Taste and adjust the seasoning if needed.
Remove the bay leaf and discard it.
Serve the Marmitako hot, garnished with fresh chopped parsley. You can accompany it with crusty bread or a slice of rustic Basque country bread.