Preheat the oven to 220C/425F/Gas 7.
Place the sweet potato halves in a pan of boiling salted water. Bring back to the boil, reduce the heat and simmer for five minutes.
Drain the sweet potato. Sprinkle each half with ground cumin, then wrap each half in aluminium foil. Transfer directly to the oven shelf and roast for 12 minutes, or until tender.
Meanwhile, mix together the yoghurt and the mint in a bowl and season generously.
To serve, open the sweet potato foil parcels and drizzle over the minted yoghurt.