1. Clean, wash and soak the black beans in enough water for 6 to 8 hours and drain well.
2. Add 5 cups of water, tomatoes and garlic and pressure cook for 2 whistles.
3. Allow the steam to escape before opening the lid and keep aside.
4. When cool, blend in a mixer till a smooth.
5. Do not strain the mixture. Keep aside.
6. Heat the butter in a deep non-stick pan, add the onions and saute till the onions turn translucent, while stirring continuously.
7. Add the black bean puree and salt and simmer for a another 3 to 4 minutes, stirring once in between.
8. Serve hot with toasted bread.