Heat the oil in a large frying pan on medium high heat and fry the onions until they are limp and tender. Add salt and pepper and then add the garlic. Add the tomatoes and cook over medium heat until the juice has reduced.
On a lightly floured surface, roll out the puff pastry and cut it into six squares: 10 x 10cm.
Place 1 full tablespoon of the filling in the centre of a square and fold the dough over to form a turnover. Firmly pinch the edges together using the tines of a fork.
Repeat the procedure with the remaining squares and place the turnovers on a baking tray.
Brush the turnovers with the egg yolk and slit the top with a sharp knife.
Place the tray in the preheated oven and bake until the turnovers are nicely golden, about 25 minutes.