Preheat the oven to gas 6, 200°C, fan 180°C. Wrap the sweet potatoes in tin foil and place them directly onto the oven shelf. Bake for 45 minutes. Meanwhile, heat the oil in a pan, add the onion and mushrooms, cover and cook for 10 minutes, stirring occasionally. Remove the lid, stir in the green curry paste and cook for 30 seconds, then add the coconut milk. Bring to a gentle boil and simmer for 5 minutes. Just before serving, stir in the fish sauce (if using), lime zest and juice of half the lime. Add the chilli (if using) and herbs, reserving a little of both. When the potatoes are cooked, remove the foil and split down the middle. Spoon over the curry sauce, top with the chilli and herbs and serve with the remaining lime cut into wedges to squeeze over.
