1. Heat the oil in a nonstick frying pan over a medium flame and sauté the onion, carrot, and celery for approximately 10 minutes, or until they change color. Turn off the flame and set aside.
2. In a bowl, stir together the flour, baking powder, and salt for the dough. Make a well in the center and add the oil and milk. Stir with a fork. Set aside.
3. Mash the tuna with the mayonnaise. Add the egg. Add the sautéed veggies, salt, and pepper to the tuna and mix well. (If you prefer it very smooth, do this in the food processor with a few on/off pulses. Don’t overdo it or it’ll liquefy.)
4. On a floured surface, roll out the dough into a thin rectangle. Spread the filling all over the dough. Roll up jelly roll-style. Cut into slices approximately half an inch (one and a quarter centimeters) thick and place on a cookie sheet lined with Gefen Parchment Paper.
5. Alternatively, place a thick strip of tuna mixture in the center of the rolled-out dough. Fold over each side towards the center. Gently turn over and place on the cookie sheet.
6. If desired, brush the tuna roll-ups or “knish” log with egg prior to baking. Bake at 375 degrees Fahrenheit (190 degrees Celsius) for 15 minutes for the roll-ups and 20 minutes for the log. You don’t want them too brown because you want to be able to see the spiral effect. If, however, you like them crispier, bake them a little longer.
If you have leftover dough, make pizza roll-ups. Spread the dough with marinara sauce, sprinkle on grated cheese, roll up, slice, and bake. If you have leftover filling, add some bread crumbs and panfry the mixture in a little oil and voila: you have tuna latkes!