Return the beans to the pot. Add 31/2 cups of the broth, the onion, cumin, garlic, bay leaf, and olive oil. Bring to a boil, reduce the heat to a simmer, and cook, stirring occasionally and adding more broth if the broth reduces too quickly, until the beans are soft but not falling apart and the broth has reduced almost entirely, about 70 minutes. Remove and discard the bay leaf, season to taste with salt, and serve.