Creamy Curried Beans (Dal Makkhani)

This recipe is not original and I have not tried it. I have lost the source

2 c. cooked black beans
1 c. cooked kidney beans
4 t finely grated ginger
4 cloves garlic, minced
2 ripe tomatoes, finely chopped
2 t ground cayenne pepper
2 t salt
1/3 c. butter
1/2 c. half-and-half
2 t garam masala

Place half the beans in a bowl. Using a potato masher, beat or mash until they form a coarse puree. Place mashed and whole beans in a large pot. Add the ginger, garlic, tomatoes, cayenne, salt, butter, cream, and garam masala to the beans. Cover with just enough water to cover beans. Cook for 30 minutes.

To serve, sprinkle bowl of Dal Makkhani with finely diced purple onion and chopped cilantro, if desired