1. Sauté the onions and chiles in the oil until tender, about 10 minutes.
2. Add the cumin and coriander and sauté for a minute, stirring constantly.
3. Stir in the tomato and flour and cook for about 2 minutes.
4. Finally, stir in the yogurt.
5. Heat the sauce thoroughly, but do not allow it to boil or the yogurt will curdle.
6. Add salt to taste and serve.