Collard Greens with Fennel and Orange Butter

Melt the butter in a large enameled cast-iron casserole. Add the fennel and orange zest and cook over moderately high heat, stirring frequently, until the fennel browns around the edges, about 10 minutes. Reduce the heat to moderately low and cook, stirring occasionally, until evenly browned, about 12 minutes longer. Add the orange juice and boil over high heat until reduced by one-third, about 8 minutes. Stir in the collards in batches, adding more as the leaves wilt. Cook the collards until just tender, about 10 minutes. Season with salt and pepper and serve.