Give the lemons a thorough wash and dry. Remove the seeds and cut them into thin pieces.
In a bowl, combine salt, turmeric, red chilli powder and fenugreek seeds. Adjust the spices to your taste preference.
Toss the lemon pieces in the spice mix until they are evenly coated.
Transfer the spiced lemons to a clean, dry glass jar. Seal it tightly and place it in a sunny spot for a few days, shaking the jar daily to mix the ingredients.
After a few days, heat mustard oil until it smokes and allow it to cool. Ensure that all of the lemons in the jar are submerged by pouring the cooled oil over them. Give the pickle two to three weeks to mature before consuming.