1. In a saucepan put the beans to boil with the onion. When their skins wrinkle, add 1 tablespoon butter. When almost done, add salt to taste and continue cooking until the beans are very soft. Pour the broth into another container.
2. Mash the beans with a wooden spoon, add 1 cup of the broth and sauté the mixture in butter until it thickens into a heavy paste that can be rolled up. Serve hot, rolled and decorated with cheese and triangles of fried tortilla.