Soak the whole black urad dal and rajma in plenty of water for 8 hours or overnight.
Drain and rinse the soaked beans.
Add them to the Instant Pot with 4 cups water and ½ teaspoon salt.
Pressure cook on High for 30 minutes.
Allow a natural pressure release for 15–20 minutes.
Turn on Sauté mode. Add oil and 1 tablespoon butter.
Cook the onion until soft and lightly golden.
Add ginger-garlic paste and cook for 1 minute.
Add tomatoes and cook until softened and thickened.
Add chili powder, turmeric, coriander powder and cumin powder. Stir well.
Add the cooked dal and rajma to the masala.
Mash some of the dal with the back of a spoon or potato masher.
Simmer uncovered for 15–20 minutes, stirring occasionally.
Add remaining butter, garam masala and kasuri methi.
Adjust salt and consistency with hot water if needed.
Garnish with fresh coriander and serve hot.
For an optional smoky flavor, finish with a brief dhungar.