Preheat the oven to gas mark 6 (200C). Brush a baking sheet with a little oil and season lightly with salt and pepper.
Arrange the un-skinned white fish fillet on the baking sheet, skin side down. Sprinkle the fish with the herbs and dot with butter or splash with the oil (optional). Season lightly with salt and pepper.
Arrange the dark fillet on top, skin side upper most. Season the skin with rock salt and freshly ground black pepper.
Roast in the oven for 10-12 minutes or until the fish has lost its translucency and the skin will easily pull away from the fillet.
Lift onto a serving dish, pull the skin away and cut across the middle of the fillet and serve the lemon wedges and salad to squeeze over the fish.