Heat the olive oil in the soup pot. Add the onion, carrot and celery and cook over moderate heat, stirring occasionally, until the vegetables are softened, about 8 minutes. Stir in the garlic, bay leaf and 1 teaspoon each of the chopped thyme and rosemary and cook until fragrant, about 2 minutes. Add the drained beans, stock and 3 tablespoons of the reserved bacon fat and bring to a boil. Simmer the soup over moderately low heat until the beans are tender, about 1 1/2 hours.