Heat 2 tablespoons of the oil in a large roasting pan and fry the shallots and pancetta, stirring frequently, until beginning to colour. Add the pine nuts and chopped rosemary with a little pepper and fry for a further 2 minutes. Drain to a bowl, add the breadcrumbs and anchovies and mix well.
Add the onions to the pan and fry for 5 minutes until slightly softened. Stir in the tomato wedges and remove from the heat. Push to the edges of the pan to leave a space for the fish in the centre.
Check the fish for any stray bones and place one fillet in the pan. Pack the stuffing mixture on top, pressing it firmly on to the fish with your hands. Lay the second fillet on top, skinned-side down, and season with a little salt and pepper.
Drizzle with the remaining oil and place in a preheated oven, 180°C (350°F), Gas Mark 4, for 30 minutes until the fish is cooked through. Test by piercing a thick area with a knife.