In a large bowl, whisk together yogurt, gram flour, and turmeric (if using) until smooth. Set aside.

In the jar of a blender, blitz together garlic, chilies, and 2 teaspoons cumin seeds with a bit of water into a smooth paste. Add paste to whisked yogurt mixture. Add 4 cups water to mixture and set aside.

Heat oil in a large vessel over medium high heat. Add onions, and once they begin to burn around the edges, add the mustard and fenugreek seeds.

Once the onions are golden brown, add remaining 2 teaspoons cumin seeds and curry leaves and cook for a minute.

Gently add yogurt mixture and salt, and bring to a boil. Reduce heat to medium low, cover the pot with a lid and leave it slightly ajar (this prevents the karhi from boiling over). Cook for 45 to 60 minutes, until the oil has risen to the top and the karhi has slightly thickened. Remove from heat and garnish with cilantro.