Put the mangoes in a ceramic or stone jar or bowl and set aside.
Rub the mustard seeds between the palms of your hand to try to remove the dark musk as much as possible.
Then put them in a mortar or a spice grinder and pound with a pestle or process until ground.
Spread the salt and ground chillies out on clean tea (dish) towels and let them dry so there is no moisture left.
Mix the mustard powder, ground chillies and salt together in a large bowl, adding a little water to make a paste.
Add the paste and fenugreek seeds to the mangoes in the jar or bowl for 12 hours and leave them with a cloth covering the jar or bowl until the mangoes release water.
Drain out the water gently.
After draining out the water, pour in enough oil to stand about 8-10cm / 3-4 inches above the mangoes.
Mix the spices and mangoes well with a dry wooden spoon and make sure there is still 10cm / 4 inches oil on top of the mangoes mixture.