Wash the mangoes and then wipe them with a kitchen towel. Cut 1 inch slices of mango. Cut the seed as well. Discard inner part of the seed, but keep the outer shell that is attached to the mango piece.
Add 3 teaspoons of rock salt and 3 teaspoons of turmeric powder to the mango slices. Mix well. Keep them in sunlight or in a corner for 3 hours.
In the meantime, heat mustard oil to smoke point, and then allow it to cool to room temperature.
Roughly grind all the whole spices, that is fennel, fenugreek, mustard, and onion seeds together in a grinder. Mix all other spices to these grounded spices. Add half of the mustard oil to these spices, and mix well.
After three hours, remove all the juice secreted by mango slices. Mix spices with these slices. Put these slices in a sterilized glass jar or jar made of any other neutral material.
Pour rest of the mustard oil over the mango slices. Cover the mouth of jar with a clean cotton cloth or lid. Keep the jar in a warm place or sunlight. Stir the pickle everyday using a clean dry spoon.
After four days check the pickle. Gently press the slices so that all of them are submerged in oil. If needed add some more oil. Cover the jar, and store it in a warm place once again. Pickle will be ready in a week to 15 days depending upon weather and quality of mangoes. If the mango pieces have softened a bit, that means pickle is ready.
Utensils, jar, and spoons used in making pickle should be clean and dry.
Always handle pickle with clean spoon. If handled properly, pickle remains good for about a year.