1. Wash potatoes and place in a 5-quart pot with water to cover. Bring to a boil, reduce heat, and simmer until potatoes are tender (about 30 minutes). When potatoes are done, remove from pot, cool, and save the water. Peel potatoes, cut into cubes, and mash until all lumps disappear.
2. With a wooden spoon, stir in cornmeal, 1/2 cup at a time. Continue to add flour until mixture forms a soft dough. If dough becomes too dry, add a teaspoon or two of the water from the boiled potatoes. Roll dough into a ball, cover with a damp cloth, and refrigerate while you prepare the filling.
1. Place tuna in a small bowl. Add salt, red-pepper flakes, garlic, and vinegar and mix well. Marinate tuna for 30 minutes.
2. Heat oil in medium fry pan, add chopped onion and sauté until soft and transparent. Add tomato and tomato paste, mix well, and simmer 5 minutes or until tomato is soft. Stir in the tuna mixture and cook over low heat for about 5 minutes (or, if using canned tuna, until mixture is heated through). Add water from potatoes, as needed, so that the mixture remains moist. Remove from heat and cool.
3. Remove dough from refrigerator. Spread a sheet of plastic wrap on counter. Place 1 heaping tablespoonful of dough, about the size of a golf ball, in the center of the plastic wrap. Press with the heel of your hand to form a circle 3 to 4 inches in diameter. Place a tablespoon of filling in the center of the dough. Fold plastic and dough over filling to form half circle. Pull back plastic and pinch edges of dough to seal. Repeat with remaining filling and dough.
4. Heat 1 inch of oil in a large skillet to a temperature of 350°F (180°C) or until a test piece of pastry sizzles. Fry pastries until golden, about 3 minutes each side. Drain on paper towels and serve warm or at room temperature.