Whisk together the yogurt, gram flour, turmeric powder, red chili powder, coriander powder, and salt in a bowl. Whisk until the batter is smooth.
Heat the oil in a large pot or saucepan over medium heat.
Add the ginger garlic paste, and chopped onions and fry for a few minutes until the onions are a little golden.
Add the chopped tomatoes, and fry for a few minutes until the veggies are soft and golden brown.
Add the yogurt and gram flour mixture to the pot, and stir well.
Add in the cups of water and mix well. Bring the mixture to a boil, then reduce the heat to low and let it simmer for about 15-20 minutes, stirring occasionally, until the karhi thickens.