Preheat the oven to 200°C.
Wash and dry the sweet potatoes, then cut in half lengthwise.
Rub with oil, place in roasting dish, cover with foil.
Bake for 30 minutes, remove the foil and bake for another 15 minutes.
Cut the cheese into small cubes.
Heat the oil in a frying pan, soften the onion for about 8 minutes. Add the garlic and fry for 1-2 minutes, then remove the pan from the heat.
When the sweet potatoes are cooked through, remove from oven and carefully scrape out the flesh into a large bowl. Add the onion mixture, cheese, egg & leaves of 3-4 thyme sprigs and, using a potato masher, mash to a rough texture. Season with salt & pepper and add about ½ teaspoon of grated nutmeg. Stir well, then fill the sweet potato skins with this mixture. Sprinkle the top with a little oil.
Return the stuffed sweet potatoes to the roasting tin and cook in the oven for 15-20 minutes until golden brown.
Sprinkle with thyme leaves before serving.