In a large mixing bowl, add the raw mango cubes and sprinkle the salt over them. Mix well and set aside for about 30 minutes. This helps to extract moisture from the mangoes and enhances the pickle's texture.

In a small skillet, toast the fenugreek, mustard seeds, and optional nigella seeds over low heat until they turn slightly golden and emit a nutty aroma. Allow them to cool, then grind them into a fine powder using a spice grinder or mortar and pestle.

After 30 minutes, squeeze out any excess moisture from the mango cubes gently in a colander. Transfer the mangoes to a clean, dry bowl.

Add the Kashmiri red chili powder and ground seeds to the mango pieces and mix thoroughly, ensuring that each piece is well coated with the spice. Adjust the amount of chili powder according to your preferred level of spiciness.

Heat mustard oil or peanut oil in a small pan over medium heat for a couple of minutes until it almost reaches a smoking point. Turn off the heat and allow the oil to cool for 20 minutes to room temperature. Pour the oil over the mango mixture, ensuring all ingredients are well-coated. Stir the pickle thoroughly to combine all the flavors and spices.

Transfer the Avakaya Pachadi to a clean, sterilized glass jar or airtight container. Press the pickle down with a clean, dry metal spoon to release as many air bubbles as possible. Make sure there is at least 1 cm of space at the top of the jar to allow for expansion. 

Close the jar tightly and let the pickle mature in direct sunlight for at least 2-3 days at room temperature.

After fermenting at room temperature, the pickle can be stored under refrigeration for up to two months.