To make the marmitako, put the ancho peppers in a heat-proof bowl. Cover with boiling water and leave to sit for 15-30 minutes, depending on the thickness of your peppers.
To make the marinated tomatoes, mix the cherry tomatoes in the sherry vinegar, olive oil, garlic and salt together in a bowl and set aside while you make the marmitako.
To finish the marmitako, heat half of the olive oil in a saucepan and fry the onion and garlic until softened. Add the potatoes and tomatoes to the pan, scrape the pulp from the rehydrated peppers and add this, the diced padrons and the fresh red pepper to the pan. Pour in the Cava and boil for 1 minute, then add the fish stock. Season with salt and freshly ground black pepper and add the bay leaf. Simmer for 20 minutes, or until everything is cooked and thickened.
Heat the remaining oil in a frying pan over a high heat and sear the tuna chunks all over. Drop the tuna chunks into the broth, stir in the parsley, the smoked paprika, top with the marinated cherry tomatoes and serve.