Heat oil in a large pot over medium heat. Add cumin, ajwain and mustard seeds. Cook 2 mins.
Add onions, tomatoes, curry leaves, ginger garlic turmeric paste and cook 5 minutes.
In a bowl mix yogurt, besan and water. Use a whisk. Make sure there are no lumps. Add this to the pot. Add salt. Bring it to a boil and reduce heat to simmer. Cook to desired thickness. Don't let it boil over. Don't cover. You may leave a wooden spoon in the pot.