Add salt as needed (I used ½ tsp), red chilli powder, turmeric powder and mix well. Set this aside for 6-8hrs. Both the mango and onion would leave water.
In a pan, heat oil. Add mustard seeds, methi seeds, roughly torn dried red chilli and let them splutter. Finally add asafoetida and switch off the flame. Let this cool down completely.
Add this tempering to the prepared onion mango mixture and mix well.
Serve as a side dish for any rice or roti or paratha. Keep in an airtight glass/plastic container in fridge for upto a week.