Preheat the oven to 200ºC (400ºF)
In a pan over medium heat pour the oil and toss the onions. Cook for a couple minutes.
Toss the bell pepper, the garlic and the tomato paste. Cook for 5-8 minutes until the onions are transparent.
Add the drained tuna and take out of the stove.
Unfold the pastry sheet on a lightly floured surface. Roll the pastry sheet into a 14x12-inch rectangle. Place the pastry onto the baking sheet. Spoon the tuna filling lengthwise down the center of the pastry.
Cut slits 1 inch apart from the 2 sides of the pastry rectangle to within 1/2 inch of the tuna filling. Starting at one end, fold the pastry strips over the tuna filling, alternating sides, to cover the filling. Brush the pastry with the egg wash.
Take to the oven for 20 - 25 minutes until the pastry gets a golden color.