Using a medium size saucepan, heat the olive oil and cook the onion on a low to moderate heat for five minutes. Add a splash of water if the pan dries and as the onion starts to soften replace the lid and turn the heat down to cook the onions for around 20 minutes.
When the onions are fully softened stir in the sugar and salt allowing it to dissolve. Turn off the heat and set aside.
Unroll the puff pastry and cut 12 circles with the round cutter. Lay them on the baking sheet lined with baking parchment paper. Paint the edges of the pastry circles with milk and spoon a dessert spoon of caramelised red onion into the centre of each circle. Bake for 12 minutes until lightly golden.
Remove the tartlets from the oven and reduce the oven temperature to 180c / Gas 4. Dot 2-3 cubes of Apetina over each tartlet and return to the oven for 3-4 minutes until softened slightly.
Meanwhile scatter rocket and slices of marinaded red pepper into a small bowl. Mix a vinagrette with the olive oil and balsamic vinegar, toss the rocket and peppers in this. Turn onto a salad platter and arrange the warm tartlets on top. Serve immediately.
Notes
Variations:
You can buy ready-made caramelised onion jam to save time.
You can use ready-made vol-au-vent cases rather than cut out circles of puff pastry.