Grind both mustard seeds to a fine powder. Add three tablespoons water and whisk well.
Add mangoes and the mustard paste in sterilized jar. Add salt and mix well.
Heat oil in a non stick pan. Add asafetida, turmeric powder, red chilli powder and mix well.
Allow to cool. Add this to mangoes and mix well.
Let the pickle marinate for two to three days. Refrigerate and use as required.