You need a kilo of limes to start with. Wash them well and pat dry with a dish towel. Now spread them out on a tray to dry completely.
In the meantime, make your stuffing. First you mix the whole spices and grind them. You need a tbsp of black peppercorns, 2 tbsp cumin seeds, 4-5 pods of black cardamom and 8-10 cloves. Once these spices are ground to a fine powder, mix in 100 grams salt, 3 tbsp granulated sugar, 2 tbsp rock salt and 1 tsp ground cinnamon. Finally, add in 4 tbsp of the most critical spice - ajwain (also called carom seeds or bishop's weed).
Slit each lime into four, keeping the base intact so the pieces still stay together. Fill with as much spice stuffing as you can fit in a lime (1-2 tsp usually does it). Arrange these limes in a glass or a ceramic jar. When you have only 3 limes left, pour any spices you have left in the jar. Juice the last 3 limes and add that juice to the jar.
Now cover your jar and put it out on the porch, or any place you get direct sunlight. Leave the jar in the sun for two days, then let it be for another 8-10 days.
At the end of two weeks, the spices and the lemons mingle into a delicious pickle. My mom then puts it in the fridge where it keeps for at least 6 months.