Heat a pan over medium heat and add coriander, cumin, fenugreek and peppercorns and roast for two minutes until they change colour and become aromatic. Remove from heat and spread onto a plate to cool.
Add dry red chillies to the pan and roast for about three minutes, stirring until they change colour and they get a pungent smoky aroma. Remove from heat and set aside on the plate.
Add the channa dal to pan to the pan. Keep stirring while roasting for about four minutes. Channa dal takes longer to cook than the spices and when roasted the dal should look browned or golden. Remove from heat and set aside on the plate.
Add the urad dal to the pan. Keep stirring while roasting for about three minutes, or until they become golden and you get the aroma of the roasted dal. Remove from heat and set aside on the plate.
Add the curry leaves to the pan and roast until they become crisp. Remove from heat and set aside on the plate.
Add mustard seeds to the pan and when they start to pop, quickly remove them from the heat. Set aside on the plate.
Turn off the flame and add the asafoetida to the pan. Keep stirring until it changes colour and becomes aromatic - this should be less than one minute. Quickly remove from heat and set aside on the plate.
Allow the roasted spices to cool to room temperature.
Add the turmeric to the cooled whole spices. Use a grinder or mortar and pestle to grind into a fine powder.
Store in an airtight jar.
notes
This spice powder will keep for several weeks in a small airtight jar before losing its flavour.