1. In a blender put the beans with 1/2 cup cooking liquid or stock and reduce to a purée.
2. In a saucepan heat the shortening and sauté the onion until transparent. Add the beans and cook, stirring with a wooden spoon, until a soft paste. Season and set aside.
3. Peel the plantains. Put them in a saucepan with salted water to cover, bring to a boil, reduce heat and simmer 30 minutes. Drain and mash with a fork. Add grated cheese and 3 tablespoons of butter. Stir in the eggs. Butter an oven dish and spread half the banana mixture, cover with the bean mixture, and top with the rest of banana mixture. Dot with butter and bake in a 375º oven 30 minutes.