Step 1: Preheat the oven to 200°C.
Step 2: Place the bread cubes on a baking tray, drizzle with a little of the Olive Oil and bake until golden brown. Allow the bread to cool, then process in a food processor – you will need ¾ cup breadcrumbs. Reset the oven temperature to 220°C.
Step 3: Gently sweat the onion and thyme in a frying pan in a little Olive Oil until the onion is translucent. Add the pancetta and sorrel leaves to the pan, then stir in the breadcrumbs and season with salt and pepper.
Step 4: Put the fish on a baking tray and squeeze lemon juice into the cavity, then season and stuff it with the breadcrumb mixture. Brush both sides of the fish with olive oil, then squeeze over more lemon juice and season. Bake for 15 minutes, then carefully turn the fish over and cook it for another 10 – 15 minutes. Remove the fish from the oven and allow it to rest for 10 minutes before serving.