Heat oil in a pan and add mustard seed and let it pop.
Add the chopped onions and sauté till it turns pink.
Add the tomatoes, a pinch of salt and cook till the tomatoes soften and turn mushy.
Add the ground chettinad masala paste, salt and mix well. Do not add water.
Close with a lid and cook for 5 to 7 minutes over low flame. Mix every 2 min to prevent the masala from sticking to the bottom of the pan.
Add the panner cubes, chopped coriander leaves and mix well.
At this stage if the gravy is very thick, add 3-4 tbsp water.
Cook for 5 mins over low flame till the paneer gets coated with the masala paste and the flavor merges well.
Chettinad paneer pirattal is ready.
Serve hot with phulkas or chapthis.