Prick and bake all potatoes until soft, 200°C for 1hr. Scoop out flesh, roughly mash and set aside.
Heat oil in a pan over a medium heat. Add the chillies, ginger, 1 ½ tsp mustard seeds, turmeric and a good pinch of salt and pepper and cook, shaking frequently, until it smells good and pops. Pour spices over and gently mix. Taste and season if needed. Add juice of 1 lime, onions and coriander. Mix and gently and set aside.
Add flours to bowl with bicarb, 2 ½ tsp mustard seeds and a good pinch of salt. Gradually whisk in enough water (400ml) for a loose batter.
Heat oil in a pan over medium heat, pour in a ladleful and tilt pan to spread like a pancake. As the moisture on top starts to cook away and lots of bubbles appear, gently spread heap of potato filling across. When base is crispy, loosely roll up the dosa in the pan serve with yoghurt, chutneys and wedges of lime. Repeat until all are cooked.