Soak 250 grams of rajma (kidney beans) in water for 8-10 hours or overnight. Drain the water and rinse the rajma thoroughly.
Pressure cook the soaked rajma with 1.5 litres of water and 1 teaspoon of salt for 4-5 whistles or until they are soft but not mushy. Drain the water and let the rajma cool down.
In a large mixing bowl, add the cooked rajma, 1 finely chopped onion, 1 finely chopped tomato, 1 finely chopped cucumber, and 1 finely chopped green chilli.
Add 1 teaspoon of chaat masala, 1/2 teaspoon of black salt, 1/2 teaspoon of red chilli powder, and salt to taste. Mix well.
Squeeze the juice of 1 lemon over the salad and mix again.
Garnish with chopped coriander leaves and serve chilled or at room temperature.