Whisk the yogurt. In a large bowl or jug, whisk yogurt with a hand whisk or blender until it turns smooth and lump-free. Doing this first, before adding any water, breaks down the yogurt into an even base. If you dilute it while it is still lumpy, those lumps get harder to smooth out later.
Add the water. Pour in chilled water, whisking continuously, until the mixture reaches a thin, pourable buttermilk consistency.
Mix in the sattu. Add sattu and whisk well, breaking up any lumps, until the flour is fully dissolved and the chaas looks pale and slightly frothy.
If, by any chance, your sattu forms lumps, then pass the mixture through a fine sieve.
Season it. Stir in roasted cumin powder, black salt, hing, 1ginger, and green chili. Taste and adjust the salt and spice to your liking.
Finish and chill. Stir in onion, mint, and cilantro. Refrigerate for 15–20 minutes so the flavors settle, or serve immediately over ice if you are short on time. The short chilling time lets the spices and herbs infuse into the drink instead of tasting separate from it.
If you don't like the taste or the cilantro coming into your mouth, strain the chaas through a strainer. But make sure to rest it for 15-20 minutes for the flavor of cilantro and mint to seep in.