Warm the oil in a 5-qt sauté pan over medium heat. When hot, add the onions and sauté about 7 minutes (they should just start to pick up a tiny bit of color). Add the bell pepper and sauté another 2 minutes. Add the garlic and sauté an additional minute before stirring in the salt, pepper and paprika.
Add the potatoes to the pot and stir to mix well. Add about 1 ½ cups of water to the pan -- just enough to cover barely cover the vegetables. Then stir in the tomato paste until it dissolves.
Bring everything to a simmer and cook uncovered for 20 minutes, stirring occasionally. Cook another couple of minutes if the potatoes are not tender.
Add the tuna cubes to the sauté pan and stir gently but well to combine everything. Cover the pan and simmer for 10 minutes. Serve at once.