Cut mangoes into one centimetre dices. Grind ginger to a paste. Mix the salt, sugar and mangoes in a wide mouthed bowl and leave it in the sun for a couple of days (upto four days is normal) or till the mangoes are tender and the sugar forms a syrup.
Add ginger paste, red chilli powder, mustard powder and mustard oil to the mango pieces with the syrup and cook on low heat till it reaches a one-thread consistency. Remove from heat. Cool. Store in an airtight container.