Prick the sweet potatoes with a fork, then put each one on a piece of foil. Drizzle with a little oil, season, then tightly wrap them. If you have a couple of smaller potatoes, wrap them together so they cook evenly.
Carefully place directly onto the coals of a hot barbecue and cook for 35-40 mins until partly blackened and completely soft inside. To test, insert a skewer through the foil into the centre of the potato – it should go in easily. Alternatively, heat oven to 200C/180C fan/gas 6. Put the potatoes on the top shelf of the oven and roast for 45 mins, then heat the grill to its highest setting and grill for 5 mins, turning halfway, until slightly charred.
Put the potatoes on a large platter and lightly mash with a fork, skins and all – they will fall apart easily. Drizzle over the lime juice and a good glug of extra virgin olive oil, and season well before topping with the spring onions. Serve with lime wedges on the side.