In a stockpot, heat the olive oil over medium heat. Saute the onion for about 3 minutes, just until it begins to soften. Add the green pepper, and cook for another 3 minutes. Add the potatoes and Biscayne Sauce or paprika, mix well, and then add the water. Bring the mixture to a boil over medium-high heat, stirring now and then, until it boils. Reduce the heat, and cook the stew at a low simmer, uncovered, for about 20 minutes, until the potatoes are fork-tender. Add the tuna to the stew, season to taste with salt, and cook gently for 3 to 4 minutes, just until the tuna is opaque throughout. Transfer the stew to a casserole, and serve.
Recipe Courtesy of Teresa Barrenechea, author The Basque Table