In a medium saucepan, cook the onion and garlic in the oil over moderate heat, stirring, until softened, about 5 minutes. Add the tomato and cook just until it starts to break down, about 3 minutes. Add the Chicken Stock and beans and bring to a boil. Reduce the heat to moderately low, cover partially and cook until the beans are tender and the liquid is slightly thickened, about 1 1/2 hours; add water if the beans are dry. Season with salt and white pepper.