Heat Indian sesame oil in a pan. Add in the mustard seeds, cumin seeds, curry leaves and the green chillies. Let the mustard seeds crackle.
Add in the onions and saute for a couple of minutes.
Add in the chopped tomatoes and the salt. Add in the sugar. Add in the spice powders on top of the tomato mixture. Mix once and cover the pan with a lid. Reduce the flame to low. Cook for 20-25 minutes stirring once in a while so it does not scorch at the bottom.
After the said time, mash the tomatoes and the onion mixture with a potato masher. The Thokku should be thick at this stage.
Add in the coriander leaves and cook for a couple of minutes more.
Check for seasoning and add in more salt if necessary.
Switch off the flame and let the Thokku cool. Once the Thokku is cool, bottle it in a dry container and use it within two days. If refrigerated, the Thokku will stay fresh for up to a week.
Serve with chapati, idli or dosai. This Thokku can be mixed with plain rice and had too.