Soak the rajma (kidney beans) for at least 8 hours or overnight. Boil for 15-20 mins, until soft yet not mushy. Set aside.
Heat oil and add the cumin seeds. When they sizzle and begin to change colour, add the onions. When they turn pink, add the garlic, ginger, and green chillies. Fry for 2-3 mins.
Add the coriander powder, turmeric, garam masala, red chilli powder. Mix well and cook for another 3-4 mins. My onion-masala mixture is red because I used red chilli powder instead of green chillies.
Add the cooked beans along with the water used to cook it in and the sliced tomato. Mix well and let simmer for 5 mins.
When the gravy thickens a bit, add salt. Mix well, bring to boil again and turn off heat.