Heat the olive oil in a large, flameproof frying pan, add the onion and garlic and cook over a medium-high heat for 3–4 minutes until golden and softened. Add the spinach and stir well for 1–2 minutes until wilted.
Drain off any excess juices from the pan and transfer the spinach mixture to a bowl. Stir in the feta and mascarpone and season well.
Rinse out the pan and add the sunflower oil. Place a tortilla in the pan and evenly spoon over the spinach mixture, then place the second tortilla on top. Press down well, then scatter over the Parmesan.
Place the pan over the heat and cook for 2–3 minutes, then place under a preheated hot grill and cook for 2 minutes until golden and the cheese has melted. Serve cut into wedges.