For the best flavour and texture, the dosa pancake batter is best made at least 24 hrs ahead. Measure the flours into a large bowl, add the milk, season with salt and add 300ml water. Whisk to a smooth batter, then cover with cling film and chill for 24 hrs or up to 5 days.
Heat oven to 200C/180C fan/gas 6. Toss the sweet potato in a drizzle of the oil and spread out on a large baking tray. Cook for 20 mins, stirring on the tray once or twice during cooking, until soft and starting to caramelise. Meanwhile, heat the remaining oil in a large pan and fry the mustard, fennel and cumin seeds for 30 secs or so until fragrant. Stir in the chilli, onion, garlic, ginger, coriander stalks and curry leaves, and cook over a low heat for 10 mins until the onion is really soft. Stir in the ground spices for 15 secs, then add 100ml water and bubble to bring all the flavours together.
Stir the sweet potato into the pan and season well. Use the back of your spoon to crush some of the sweet potato, leaving some pieces chunkier – the mixture should resemble very chunky mash. Keep the mixture warm until the pancakes are ready (or leave it to cool, then chill for up to 2 days – gently reheat in the pan or microwave before continuing).
To make the raita, put the grated coconut in a bowl, stir in the coconut yogurt, coriander, lime zest and juice, and a pinch of salt. If the yogurt is very thick, loosen it with 1-2 tbsp water. Chill until ready to serve (you can make this up to 2 days ahead too, but stir in the coriander just before serving).
Heat the oven to its lowest setting and put a plate inside ready to keep the dosas warm once you’ve cooked them. If the dosa batter has thickened in the fridge, thin it with a splash of water – it should be the consistency of double cream. Use a little oil to grease your largest frying or crêpe pan, wiping out the excess oil with some kitchen paper. Pour a ladleful of batter into the centre of the pan and quickly swirl it around to fill the surface, getting the pancakes as thin as you can. When the surface of the pancake looks almost dry, spoon a quarter of the filling down the centre. When the pancake is deep golden-brown and crisp on the underside, roll it up in the pan to encase the filling, cook for 1 min more, then transfer to the oven to keep warm while you continue cooking the remaining dosas.
Any leftover batter will keep well in the fridge for 4 days (if the batter was made the day before, don’t keep for longer than 5 days in total). Serve the dosas with the coconut raita, lime wedges and remaining coriander leaves, with your favourite Indian chutneys and pickles on the side.
