Wash the rajma under running cold water until the water runs clear. Or rinse them with tap water 2-3 times or until the water is not cloudy anymore. Soak in enough water for at least 8 hours or overnight. After the soaking time, they get lighter in color, and almost double in size and some beans may have wrinkly skin. Discard the soaking water.
Take soaked rajma along with fresh water in the pressure cooker (or instant pot). Drain the cooking water.Pressure cooker: 1 whistle on High heat then reduce to Low and cook for 10 minutes.Instant pot: Manual (high pressure) for 8 minutes.
To make rajma sundal, take curry leaves, ginger and green chili in a small grinder jar. Coarsely ground them into a paste and keep it ready.
Heat the oil in a pan on medium heat. Once hot add mustard seeds and let them splutter. Then add urad dal, saute with stirring constantly until dal is golden brown in color.
Add the prepared paste, dry red chili and curry leaves. Saute for 30-40 seconds or until the raw smell of ginger goes away.
Add boiled, drained rajma and salt. Mix well and cook for 2 minutes.
Add grated coconut, mix and cook for a minute. Rajma sundal is ready, turn off the stove.
Notes
The cooking liquid (the water drained from boiled kidney beans) is very nutritious. Do not throw away. Use that in making dal, sambar, rasam. Or use in kneading the roti, paratha dough.
For a variation, to make rajma chana sundal spicy, add a little red chili powder, coriander powder and very little crushed black pepper. It tastes too good. But traditionally spices are not added. Instead of adding a few different spice powders, just add sambar powder or rasam powder. That tastes fantastic.