In a large skillet, heat the oil. Add and sizzle the cumin seeds for 10 seconds while stirring. Add the onion and garlic and saute until the onions are soft and lightly browned, about 3 to 4 minutes. Reduce the heat to medium, add about one third of the cooked beans and 1/3 cup of the liquid, mashing the beans with the back of a spoon or if beans are already pureed, stir them into the liquid to create a coarse mash.
While the mixture is simmering and the liquid is being absorbed into the bean puree, continue adding beans and liquid in two more batches. Add cayenne, vinegar, and salt to taste. Serve immediately.
Recipe courtesy of Lorna Sass, author of Cooking Under Pressure