Scrub and prick the potatoes and sweet potatoes, then bake until soft, either in the coals of a barbecue wrapped in tin foil, or in the oven for 1 hour, or until cooked through.
Once cooked, cut the potatoes open, then scoop out the flesh and roughly mash.
Crumble the dried chilli and finely slice the fresh chilli, then peel and finely chop the ginger.
Drizzle 1 tablespoon of oil into a pan over a medium heat, add the chillies, ginger, mustard seeds, turmeric and a good pinch of sea salt and black pepper. Fry until smelling fantastic and the mustard seeds start to pop, shaking the pan regularly.
Meanwhile, trim and finely slice the spring onions, and pick and roughly chop the coriander leaves.
Pour the spice mixture over the potatoes then gently mix together with a knife. Taste and season, if needed. Squeeze in the lime juice, add the spring onions and coriander, then mix together.
For the dosa batter, add the flours to a large bowl with the bicarb, mustard seeds and a good pinch of salt. Gradually whisk in enough water, about 400ml, to make a loose batter.
Drizzle 1 teaspoon of oil into a large non-stick frying pan over a medium-high heat, then wipe around and away with a ball of 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. Delicious served with minted yoghurt, chutney and wedges of lime for squeezing over.