Preheat the oven to 200C/400F/Gas 6.
For the fish, cut slashes into the skin of the sea bass, rub all over with the olive oil and herbs and season well with salt and freshly ground black pepper. Leave to marinate for 10 minutes.
For the sweet potatoes and tomatoes, place the olive oil into a large heavy-bottomed ovenproof pan or tagine dish. Place a layer of sweet potato into the pan, followed by a layer of tomatoes and top with the garlic and thyme. Place the fish on top of the layered vegetables.
Transfer the pan to the oven and bake for 25-30 minutes, or until the fish is completely cooked through and the potatoes are tender.
To serve, divide the fish and vegetables between two serving plates.