Rinse broken barley 2 – 3 times to get rid of dirt. Soak it in clean water for 5 – 6 hours.
Boil water in a pot. Add soaked barley. Cook till it is thick like a porridge, mushy and nicely done like rice. Turn off the heat. Allow it to cool completely.
Once cooked, let the mixture cool completely. Traditionally, this is done in the evening.
Transfer raab to a earthen or ceramic pot after cooling. Cover the pot with a breathable cloth and let it sit overnight (or for 5–6 hours) in a cool, dark place.
By morning, the raab will have thickened and developed a slight tang. Add the buttermilk (chaas), cumin powder, salt, pinch of asafoetida, and chopped coriander leaves.
Whisk it nicely, adding chilled water or fresh buttermilk to reach your desired drinking consistency. The mixture should be thin and watery at this stage.
Jau Ki Raab is ready to serve.