How to Make It
In a large saucepan, melt 2 tablespoons of the butter. Add the onion and cook over moderately low heat until softened, about 7 minutes. Add the garlic and ginger and cook over moderate heat until fragrant, about 1 minute. Add the tomato paste and garam masala and cook, stirring, until glossy, about 3 minutes. Gradually stir in the water until smooth.
Drain the soaked lentils, black beans, chickpeas and red beans. Add them to the pan and bring to a boil. Add the remaining 6 tablespoons of butter and simmer over low heat, stirring occasionally, until the beans are tender and the cooking liquid is thick, about 2 1/2 hours. Stir in the lemon juice, season with salt and serve.