Drain the bean liquid and rinse. Heat a small saucepan with enough oil to coat the bottom of the pan, over medium heat. Add the onions and garlic and cook until translucent, about 4 minutes. Add the beans, a dash of oregano (if using) and water to cover 2 inches above the beans. Bring to boil over high heat, reduce the heat to low, and cook partly covered, until tender. Keep water in reserve to replenish the cooking liquid as necessary so the beans are always covered by at least 1-inch. The beans should be cooked until tender, about 1 hour. Drain excess liquid. Place the beans in a bowl and mash until pureed. Season with salt and black pepper and keep at room temperature.
To assemble the plate: Heat a saute pan over medium-high heat. Add enough oil to cover the bottom of the pan. Place the fritters in the hot oil and cook until golden brown on each side. Reduce to low flame and continue to cook until the fritter is hot throughout. Serve immediately, with a dollop of sour cream and cured red onions.
Plantain Dough: Rinse the plantains with the skins on. Place the plantains in a sauce pan and cover with hot water. Place the saucepan over high heat and cook until the skins of the plantain start to open. Drain the liquid and lay the plantains to cool. Peel the plaintains while the plantains are still warm, mash in a bowl to form a dough. If the dough is too wet add bread crumbs until workable. Divide the dough into 8 balls. Using a clean flat surface, flatten the balls into round circles 1-inch thick. Spoon 1/4 teaspoon of the black bean mixture in the center of the plantain. Fold the edges of the plantain over the center of the plantain to close the bean mixture inside. If you can't cover all the bean mix, take a piece of plantain mash and place over the beans, while forming into a 1-inch thick round circle.
Cured Red Onions: Place the sliced red onions in a small shallow container. Pour the lime juice over the onions, and season with salt. Mix well to combine the flavors. Cover and let sit in the refrigerator for 2 to 3 days.