Heat 2 tablespoons of the oil in a small frying pan (skillet) over medium heat.
Add the mustard powder, fenugreek, asafoetida, and chilli powder.
Stir-fry for about 1 minute, or until fragrant.
Set aside.
Heat almost all the remaining oil in a large, heavy-based pan over low heat.
Add the tomatoes and stir-fry for 10 minutes, or until well cooked.
Remove from the heat and allow to cool.
When cool, put the tomatoes in a blender or food processor.
Add the fried mustard powder mixture, the tamarind extract and turmeric and process until mixed together.
Transfer to a bowl.
Heat the remaining oil in a large frying pan over medium heat,
Add the tempering ingredients, except the salt, and stir-fry for about 1-2 minutes, or until dark brown.
Add the salt and allow to cool.
Then add it to the tomato mixture and mix well.
Store in a sterilized airtight ceramic jar for up to 2 weeks in the refrigerator.