Preheat oven to 200'C, gas mark 6. Scrub the sweet potatoes and pat dry with kitchen paper. Prick the skin several times with a knife, then bake on a foil-lined tray for 1 hour, or until the flesh is soft and tender.
Meanwhile, in a large bowl, mix the chick peas with 1 tbsp oil and the salt, pepper and spices. Spread evenly on a baking tray and roast in the oven with the sweet potatoes for the final 20 minutes of cooking time, stirring half way through.
Fry the garlic, onion and chilli in the remaining 1 tbsp oil over a medium heat for 5 minutes, until the onion is soft and translucent.
To serve, toss the roasted chick peas through the onion mixture. Slice open the sweet potatoes and divide the chick pea mixture between them. Top each one with a dollop of coconut milk yoghurt and a sprinkling of coriander.