Prick the potatoes and sweet potatoes all over with a knife and bake until soft, either in the coals of your fire or barbecue or wrapped in foil in the oven at 200°C/400°F/gas 6 for around 1 hour, or until soft and cooked through. Cut open on a board and scoop out the flesh into a bowl. Roughly mash then set aside while you cook your spice mixture.
Heat a lug of olive oil in a pan over a medium heat. Add the chillies, ginger, mustard seeds, turmeric and a good pinch of salt and pepper and cook, shaking the pan frequently, until it smells fantastic and the seeds start to pop. Pour the mixture over your potatoes then gently mix together with a knife – I guess it’s a bit like glorified mash potato. Taste and season if needed, then add the lime juice, spring onions and coriander. Mix everything together, then set aside.
For the dosa batter, add the flours to a bowl with the bicarb, mustard seeds and a good pinch of salt. Gradually whisk in enough water, about 400ml, to make a loose batter. Add a splash of oil to a pan over a medium-high heat and carefully wipe it around with kitchen paper. Add a spoonful of batter to the pan and immediately twist so the batter coats the base and slips up the edges. As soon as the moisture on top starts to cook away and there are lots of bubbles, add a few heaped teaspoons of potato filling and gently spread across the dosa. Once the base is crispy, loosely roll up the dosa in the pan and you’re ready to go. Serve with minted yoghurt, chutneys and wedges of lime.