Soak kidney beans overnight in lots of water. They will double in size by morning.
Wash them well and cook in a pressure cooker in water that completely covers them. Add salt. Cook for 5-6 whistles till it is well cooked. It must be of slightly mushy consistency. Save the water.
To prepare masala for curry, heat oil in a wok/pan. Add cumin seeds. Let them sputter.
Now add chopped onions and slit green chillies (optional) saute on low heat till light golden. Takes about 10 minutes. Now add ginger garlic paste.
Cook till the raw smell goes or for about 5-7 minutes. Now add chopped tomatoes.
Cook together till mushy. Takes another 7-10 minutes.
Cool and blend in a blender to make a thick masala paste.
Put it back on the fire. Add garam masala, coriander powder, cumin powder, chilli powder and dry mango powder. Add cooked rajma with reserved water.
Mix well. Lightly crush rajma beans with the back of the ladle to make the gravy creamy. Taste and add salt.
Cook on low flame for about 10 minutes.
Rajma masala gravy turns a dark red colour and is silken and creamy in consistency. Taste and adjust seasoning.
Garnish with chopped coriander leaves.
Serve hot with boiled basmati rice for a gastronomic delight.