Melt butter in a heavy saucepan over medium heat. Cook onion until deep golden, 12 minutes. Add garlic and ginger; cook 2 minutes, then stir in crushed tomatoes, garam masala, and Kashmiri chilli; simmer 15 minutes.

Add cooked lentils and beans to the masala base with 200 ml cooking liquid. Simmer uncovered on very low heat, stirring often, for 45 minutes until thick and almost creamy.

Stir in cream, adjust salt, and simmer 5 more minutes. Garnish with fresh coriander and an extra knob of butter to serve.