Wash the mangos and cut them into chunks of approximately 2 cm each. Note the skin should be left on.
Place the mango chunks into the vinegar and allow them to soak for at least 2 to 3 hours. This helps the mango absorb all the delicious masala flavours.
Combine the mustard powder, salt, chillies, and atchar masala in a little oil to create a moist mixture.
Transfer the mango atchar into a sterilized jam jar or storage container.
Then add the atchar masala mixture with the rest of the oil; stir well and cover.
Allow your mango atchar to marinate for three days; during this time, stir the mixture regularly.
Once the marinating time is complete, you can either dig in or seal your atchar tightly and store it in a cool place.
