Add sour curd to a large bowl and whisk it until completely smooth and lump-free. Gradually add singhare ka atta while whisking continuously, making sure the flour is fully incorporated into the curd.
Slowly pour in the water while continuing to whisk. Mix everything well until you have a smooth, lump-free mixture with no flour settled at the bottom.
Transfer the mixture to a heavy-bottomed pan and cook over low heat. Stir constantly for the first 5–7 minutes to prevent the curd from splitting or sticking to the bottom.
Continue cooking on low heat for another 8–10 minutes, stirring occasionally. The kadhi should become slightly thick and lightly coat the back of a spoon while still remaining pourable.
Heat ghee in a small pan and add cumin seeds. Let them sizzle, then add slit green chillies and grated ginger. Cook for a few seconds until aromatic.
Carefully pour the prepared tempering into the simmering kadhi. Add sendha namak and optional black pepper, then let it simmer for another 2–3 minutes.
Turn off the heat, garnish with fresh coriander, and serve the vrat ki kadhi hot with sama ke chawal, kuttu ki puri, or sabudana khichdi.