Prepare the Mangoes: Wash and dry the mangoes thoroughly. Cut them into small pieces, discarding the seed. Spread the pieces on a clean cloth and let them air dry for 3–4 hours to eliminate excess moisture.
Dry Roast the Spices: In a pan, dry roast mustard seeds, fennel, and fenugreek seeds until aromatic. Let them cool and coarsely grind them for a rustic texture.
Mix the Ingredients: In a large bowl, combine the dried mango pieces with salt, turmeric, red chili powder, and the roasted spice mix.
Sun Dry the Mixture: Place the mixture in a clean jar and leave it in the sun for 3–4 days, shaking the jar daily to ensure even mixing.
Add Mustard Oil: After the sun-drying period, pour in the mustard oil, ensuring the mango pieces are fully submerged.
Final Fermentation: Seal the jar and let it sit in a cool, dry place for another 3–4 days. The pickle is now ready to enjoy.