Peel ginger and fine chop in food processor or grate it manually.Repeat the same with mango.
Dry them in a bowl along with some salt ,for 4-5 hours in hot sun.
Roast mustard seeds,fenugreek and red chillies in a pan and grind them once cooled.
Now mix all the dry ingredients.
Oil the jar with 2 tablespoon mustard oil.
Add the oil to the dry mixture along with apple cider vinegar.
Mix well and leave it in sun for 1 month.Mix every 10 days with a clean dry spoon.
Notes
The chilies (whole/red chilli powder) mentioned above can be adjusted as per your taste.The vinegar mentioned above can also be omitted ,if your mangoes are very sour. Just ensure that the pickle mix has enough oil to cover it from the top.The mustard seed mix needs to be grinded just before adding to the pickle, if you have grinded the mix in advance, keep it in an air tight jar