Wash the mangoes and pat them dry with a cloth. Peel the mangoes.
Grate the mangoes in the largest setting of the grater. This should yield around 2.5 cups of grated mangoes. If the yield is less or more, adjust the other ingredients accordingly.
Take a heavy bottom kadai and heat 4 tablespoons of gingelley oil. Add the mustard seeds and once it sputters, add the asafoetida.
Now add the grated raw mangoes and 1 teaspoon of salt and mix well. Let it cook on medium flame for around 5 to 7 minutes. Keep stirring in between.
Meanwhile, if you do not have fenugreek powder, you could dry roast the fenugreek seeds, let it cool down and grind it coarsely.
Once the mangoes become translucent and the quantity slightly reduced, add turmeric powder and the remaining salt. Cook for couple of minutes on medium flame.
Now add the chilli powder and cook for another 8 to 10 minutes or until the oil starts separating. Continue cooking on a low to medium flame while stirring in between.
When you start seeing the oil to separate from the mangoes, add half teaspoon fenugreek powder and two teaspoons of grated jaggery and cook for one more minute.
Once the oil separates out completely, turn off the stove.
Cool the thokku before transferring to a clean glass or ceramic container. You can refrigerate it for a longer shelf life.