Step 1: Heat the oil in the cooker. Add the mustard, let it splutter & then add the jeera.
Step 2: Add the chopped onions & green chillies. Stir & fry for a few minutes until; the edges of the onions start turning golden.
Step 3: Add the tomatoes & cook again for a 2-3 minutes, stirring now & then.
Step 4: Now add the potatoes, salt, turmeric, chilli powder,the masala, if you are using them & 1 1/2- 2 cups of water & close the cooker. Pressure cook until 3 whistles come & switch off the gas.
Step 5: After the cooker releases, open the lid & cook again for a few minutes. Mash some of the potato pieces & continue cooking for a couple of more minutes, until the gravy thickens just a bit.
Step 6: Top with chopped coriander & serve piping hot with chawal & rotis.