Next, peel the mangoes and grind the pulp along with roughly chopped green chilies to a paste.
In a medium hot saucepan, add the mango paste, mustard powder, turmeric and salt to taste. Pour vinegar and half the mustard oil. Give it a mix and cook for just two minutes. Taste for salt.
Turn off the heat, pour rest of the mustard oil, mix and let it rest for couple of hours. Once it comes to a room temperature, pour it in airtight bottle and keep it in the refrigerator.
The spice and pungency gets better the next day.
Notes
I used a mix of yellow and black mustard seeds. However, it can easily be prepared with just black mustard seeds. If you prepare it with just black mustard seeds, add a little bit more turmeric to get the bright yellow color. Also, keep in mind that black mustard seeds are more pungent than the yellow one.
The kashundi might taste slightly bitter at the first go but as it rests, the pungency and spice comes out better.
If the mangoes are not too sour, add tamarind paste to compensate for the sourness.
If the consistency of the kashundi seems too thick, add more mustard oil and give it a mix.