Heat a large pan on the stove and add oil.
When the oil is hot, add fenugreek (methi) seeds, allow to sauté for 5 seconds or till they burst. Don't let them blacken.
Add green chilli slices, turn them so they scald all over (about 5 seconds)
Add minced onions, stir well and sauté till the onions are fried, 1 -2 minutes on low flame, stirring frequently so the onion does not catch at the bottom of the pan. Ensure the onions don't brown.
Add ginger and then garlic (paste or minced). Stir each well, sauté for about 5 seconds each, on low flame. Again ensure the garlic does not begin to brown.
Keeping the stove flame on low, add the tomato slices. Stir well.
Drain and add the potato slices, stir well. Always stir so that the onions and garlic are lifted away from the bottom of the pan as they can easily catch and char.
Add turmeric powder, red chilli powder and garam masala. Most recipes suggest adding the garam masala at the end (after adding the rajma) so that its aroma is retained in the curry, and you could do that too. I add the garam masala at this stage while making the masala. (Perhaps it is because I use a garam masala which is very aromatic and the aroma is retained even after pressure cooking the rajma masala as in the steps below.)
Again stir the entire masala well and sauté for 2 minutes on low flame.
Add about half the minced coriander leaves and reserve the rest for garnish. Stir!
Add the tomato puree, and stir.
Add the drained cooked rajma, stir briskly, (I stir till my arm aches!) turning the rajma and its masala several times, scraping the bottom of the pan so that masala does not stick. Stir for at least 5 minutes.
Transfer the contents of the pan to the pressure cooker.
Add back the water in which the rajma was cooked. You need sufficient water to just cover the rajma masala. Add more or less water depending on this. If you need to add fresh water as all the cooking water has been absorbed, ideally add warm water.
Stir once. Add salt to taste. (remember you have added a little salt while cooking the rajma, so a taste check would be helpful here).
Close the cooker, and cook on high for 2-3 whistles.
To serve: Transfer the rajma masala to a serving dish, garnish generously with minced coriander leaves. I keep lemon wedges available for those who want to squeeze some lemon juice on the rajma.
Serve with hot rice and a salad. Goes well with roti and phulka too. Or even with a lightly toasted focaccia or bread. Thick rajma without its liquid is a nice filling for a wrap or quesadilla.