Soak the dhal for 1 hour, then drain and rinse. Place in a pot with the turmeric and cover with 1 litre of water. Bring to a boil and skim off any scum that rises to the surface. Reduce to a simmer and cover with the lid slightly ajar. Cook until the dhal is tender, about 1-1.5 hours, stirring occasionally. Check regularly to make sure the dhal is not sticking and add more water if needed, only a little at a time. When almost done add salt to taste. Pulses are very bland and it will take more salt than you think. Don't salt until now as this will prevent the dhal from softening.
Make a paste with the ginger, garlic and chilli in a food processor, mortar and pestle or chop/grate finely by hand. Heat a thick pan (cast iron if you have it) and melt the ghee or oil. Add the cumin seeds and asafoetida. When the seeds start to sputter, cook carefully for a minute so as not to burn then add the onion with a pinch of salt. Cook until the onion begins to turn brown; if it starts to catch, splash with a bit of water and turn the heat down a little, this will take 10-15 minutes.
Add the ginger, garlic and chilli mixture and stir, then add the remaining dry spices, stir and cook for a minute. Remove the pan from the heat and pour over the cooked dhal. Stir in, taste and adjust the seasoning.
Reserve some or all of your dhal to dehydrate. Leave until it's cool enough to handle.
Spread the dhal onto dehydrator trays lined with nonstick sheets or parchment paper. Dehydrate at 63C/145F for 8-10 hours until totally dry. Vac pac or store in baggies until ready to use. Optionally, you can blitz your dried dhal in a food processor to reduce bulk before packing away.
Put the dehydrated dhal into a pot and cover with water.
Place pot over medium heat and bring to a boil.
Turn down the heat and cook, stirring occasionally, for about 5-10 minutes, until fully rehydrated.
Serve with rice or naan, fresh coriander, lime pickle, chutney, and yoghurt if you have it.