Slit the karondas.
Sprinkle 10 g salt on top of them and rub nicely. Set aside for 3-4 hours. Adding salt enables the karonda to be de-seeded easily.
After 3-4 hours, remove the seeds and thoroughly wash them.
Spread the karonde pieces on a fine muslin cloth.
Place the karonde pieces with the muslin cloth in the strong sun to dry for 3-4 hours.
When the karondas are slightly dry (known as फरहरा in Hindi) move them inside from the sun. Do not dry the karonde completely in sun. It is important that they should be only partially dry.
In a wok, bring mustard oil to smoking hot temprature.
When the oil is boiling, add remaining 20 g salt to split the oil (known as तेल को फाड़ना in Hindi) and turn off the gas.
Allow the oil to cool down completely.
Once the oil is completely cooled, add karonda pieces, and pickle masala. Mix vigorously.
Transfer to a large glass jar or मर्तबान/ बरनी with an air-tight seal. Close the seal.
Place the ready pickle in strong sunlight for 4-5 days. You can place the jar every day at the time of strong sun and remove them indoors at night time.
After few days, karonde ka achar (karonda achar) is ready to eat. Enjoy with mathris, poori or bajre ka rotla or just like that.