1. Sauté the onions in a pan with a little olive oil and salt. Add the bell peppers and tomatoes. When the vegetables are cooked, add a little tomato paste and the tuna. Season to taste. Simmer for a few minutes.
2. When the filling is ready, remove it from the heat and let it cool (if you put the hot filling on the dough, it will melt!).
3. Meanwhile, cut the puff pastry into medium triangles. When the filling is cold, place half a tablespoon of filling in the center of the triangle and fold the three corners to the center to form a small triangular turnover.
4. Shape all the turnovers and place them on a parchment paper on the baking sheet. Brush them with egg and sprinkle sesame seeds on top before baking - separately or far apart from each other to prevent them from joining or sticking together during cooking.
5. Bake at 180°C (350°F) for 5 to 8 minutes (depending on whether you prefer them to be white or golden).
6. There you have it, you will have about 15 small turnovers.