Stir together butter, fennel seeds, lemon peel, pepper, and salt.
Cut off and discard upper stalks of fennel bulbs. Remove any wilted outer layers; cut off a thin slice from the fennel base. Wash fennel; cut into quarters. Remove cores. Cut fennel lengthwise into 1/4-inch-wide strips.
Place beans in a 4-quart Dutch oven. Cook beans, covered, in a small amount of boiling salted water 4 to 5 minutes. Add fennel strips to beans. Cook 6 to 10 minutes more or until vegetables are crisp-tender. Drain. Toss fennel and beans in the bowl with the seasoned butter. Transfer to a serving dish.