Heat a pan with oil. Add cumin seeds, when it sizzles, add onion and cook till it becomes translucent.
Add chopped tomatoes, 2 pinch salt and cook till the tomatoes break and become mushy.
Add the ground paste and mix well. Add 1/4 cup of the retained rajma cooked water to the mixer jar, rinse and add it to the pan. Add salt, mix well and close with a lid.
Let it cook for 3-4 minutes over low flame till the raw smell of the paste disappears and everything comes together.
Add cooked rajma, enough rajma cooked water to the consistency you prefer and mix well.
Close with a lid and let it cook further for 5-7 minutes over low flame. Mix in between to avoid the gravy from sticking to the bottom of the pan.
Finally sprinkle garam masala powder and coriander leaves. Mix well, switch off stove and close the pan with the lid and let the rajma sit for 3-5 mins for all the flavors to imbibe.
Serve this rajma masala with flaky parathas, rotis, jeera rice or to make rajma chawal.