Rinse the rajma beans for a couple of times in running water.
Then soak the rajma beans in enough water overnight or for 8 to 9 hours.
Rinse the rajma beans again in fresh clean water. drain them and keep aside.
In a pressure cooker add the rajma along with the chopped onion, tomatoes, ginger, garlic and green chilies
Now add the spices - cumin seeds, red chili powder, turmeric powder and salt.
Pour water and stir very well.
Add butter or oil. Stir again.
Cover tightly and pressure cook for 18 to 20 whistles on a high flame.
Once the pressure settles down on its own, open the lid and check the beans. If the beans are still not cooked completely, then add about ½ cup more of water and pressure cook again for 5 to 6 whistles.
Once the rajma is cooked well, then check the gravy or curry also, you will see thin water like stock with the tomatoes and onions floating. so you have to continue to simmer the rajma masala till the curry thickens a bit.
keep the cooker on stove top and on a low to medium flame, simmer without lid. stir at intervals.
when you keep the curry on the stove top, then add dry mango powder. also add garam masala powder.
stir well and simmer the curry till the consistency thickens. mash a few rajma beans with the back of a spoon to thicken the consistency.
when the consistency of the curry is no longer broth like or water like and thickened. then the rajma masala is ready. if you want you can thicken the gravy more, if having with chapatis or phulkas.
lastly check the taste and add more of salt, garam masala, red chili powder or dry mango powder if required.
garnishing rajma masala with coriander leaves is optional. serve rajma masala with some steamed rice or with chapatis, parathas or bread.