Place a pressure cooker on the stove and add oil. When the oil turns hot add mustard seeds and splutter
Add onion and cook for 2 minutes
Furthermore, add green chilli and cook for 2 more minutes
Add red chilli powder, turmeric powder and mix well
After the spice powder is well mixed. Add tomatoes, salt, and a pinch of sugar and mix well. no need to add water
Let the mixture comes to a boil. Mix once and make sure no spice powder is stick to the cooker
Close the cooker and cook for 20 minutes (23-25 whistles). Once steam releases naturally, open the cooker lid and mix well
Add besan flour and mix well and cook
Finally, add coriander leaves and cook till the thooku comes together; switch off and serve with tiffin varieties like idli/ dosai/ chappathi