Place the sweet potatoes on the lower rack of the halogen oven. Brush with the butter and season lightly with salt and pepper.
Set the temperature to 250°C (482°F) and cook for about 40 minutes until tender. Leave until cool enough to handle, then cut in half lengthways and scoop the flesh into a bowl, leaving a 5 mm (¼ inch) thick shell. Mash the flesh with a potato masher until smooth.
Add the spring onions, oregano, parsley, garlic, goats' cheese, any remaining butter and plenty of pepper to the bowl and mix well. Pile back into the shells.
Return to the lower rack of the oven and bake for a further 10 minutes or until very hot. Serve as a light lunch scattered with finely chopped lemon thyme or coriander to garnish.