Preheat the oven to 200˚C/ fan 180˚C / gas 6. Cut the sweet potatoes into thick wedges, place in a large roasting tin, drizzle with 2 tsp of the groundnut oil and turn to coat well. Season with freshly ground black pepper and roast for 45 minutes, turning occasionally until tender and lightly charred.
Meanwhile, combine the piri piri sauce, fresh thyme, sugar and the remaining groundnut oil. Slash the sides of the fish and place in a flat non-metallic dish. Spoon over the piri piri marinade, gently rub in and set aside for 15 minutes.
To make the soured cream sauce, combine the soured cream, lime juice and coriander in a bowl, mix and set to one side.
To barbecue, place the fish skin side down on the grill and cook for 3-4 minutes until lightly charred. Turn over and cook for a further 3-4 minutes or until cooked through. Serve with the sweet potato wedges, a lime wedge, the soured cream sauce and a crisp green salad.
Chef's tip: once the sweet potato wedges are cooked, place in a double layer of foil and keep warm on the barbecue grill.