Place beans, onion halves, carrot, thyme and rosemary sprig and bay leaf in a large pot; cover the beans with 1500 ml of water. Crumble in the vegetable cube, cover and bring to a boil. Reduce the flame; simmer about 30 minutes over low heat until the beans are nearly tender. Drain the beans and reserve the cooking liquid. Discard all of the other solids. Set aside.
Sauté sliced onion and garlic in butter and olive oil for 6 minutes over moderately high heat, stirring occasionally.
Add the collards and cook, tossing, until slightly wilted.
Add half of the reserved bean cooking liquid, cover and cook over low heat, stirring occasionally, until the collards are tender, about 50 minutes.
Add the beans, sausage and the remaining bean cooking liquid to the pot and cook for 15 minutes. Just before serving, stir in the lime zest and juice. Season with pepper.