In a medium bowl, whisk the yogurt until completely smooth. Gradually pour in 2 cups / 480ml of water, whisking continuously to create a thin, lump-free mixture.
In a small bowl, make a smooth paste by mixing the rice or chickpea flour with a tablespoon of water.
Add this paste to the diluted yogurt along with the turmeric, sambar powder, and salt.
Whisk everything together until well combined.
Heat the oil in a small saucepan over medium heat.
Once hot, add the asafetida, followed by the mustard seeds.
When the mustard seeds begin to pop, add the fenugreek seeds and stir once.
Carefully pour the yogurt mixture into the saucepan.
Stirring constantly, bring the sauce to a gentle simmer over low-medium heat. Be careful not to boil it.
Let it simmer for 2-3 minutes, continuing to stir, until it thickens slightly.
Remove from heat and stir in the chopped fresh cilantro and optional green chili.
Serve immediately in small bowls.