Grind the mustard to a fine powder in an electric spice or coffee grinder
Discard the mango juices and squeeze the mango pieces to extract as much joiuce as you can. Discard this also.
Lay the mango pieces on a chopping board and pound them with a pestle to roughly break them up
Add the mustard, turmeric and sugar if using it plus extra salt to taste
Knead the mixture by hand as you would for bread dough for a few minutes. It's a good idea to wear food gloves for this step.
Finally add a good glug of mustard or other vegetable oil and mix in with a spoon
Place the mixture in a clay pot with a loose fitting lid. The clay will absorb any excess moisture and help guarantee the mixture won't rot before fermenting. Covver the whole thing with muslin and leave at room temperature for about a week
Transfer to a clean glass jar with a tight fitting lid. Store in the fridge for up to three months, during which time it will improve