Using a mortar and pestle, coarsely crush the ginger, black peppercorns, cloves, fennel seeds, and cinnamon stick to release their natural oils and aroma.

Pour the water into a saucepan and add the crushed spices along with the tulsi leaves.

Bring the mixture to a boil on medium heat. Reduce the flame and let it simmer until the liquid reduces to about half. Boil for 3 to 4 minutes, then simmer for another 2 to 3 minutes.

Turn off the heat, strain the kadha, and add jaggery while the decoction is still warm. Stir well and serve.