Heat the lard and fry the onion, without browning, until it is soft. Add 1 cup of the beans and their broth to the pan and mash them well over a very high flame. (With a bean masher or wooden potato masher) Add the rest of the beans gradually, mashing them all the time, until you have a course puree. When the puree begins to dry out and sizzle at the edges, it will start to come away from the surface of the pan. As you let it continue cooking, tip the pan from side to side. The puree will form itself into a loose roll. This will take from 15 to 20 minutes. Tip the roll, rather like folding an omelet, onto the serving dish and garnish with the cheese. Spike it with the crisp triangles of tortillas. Decorate.