Heat oven to 200C/180C fan/gas 6. Toss the sweet potatoes with chilli, 1 tsp oil, cumin seeds, garam masala and plenty of seasoning on a roasting tray. Roast for 25-35 mins, turning wedges halfway, until tender and golden.
Meanwhile fry onion in remaining oil for a few mins until partly softened, stir in the lime juice and sugar with some seasoning, then turn off the heat.
Warm chapatis following pack instructions, then assemble by adding a couple of wedges of sweet potato to each, followed by a scattering of onions, a dollop of yogurt and a small handful of coriander leaves. Roll up and eat.