Preheat oven to 450°. In large saucepan of boiling salted water, cook potatoes over moderate heat 10 minutes. Add beans, cover partially; cook until beans and potatoes are quite tender, 5 to 10 mintues longer. Drain well.
Transfer potatoes and beans to food processor and puree, in batches if necessary, until smooth. With machine on, add 4 tablespoons butter, 1 tablespoon at a time, nutmeg, salt and pepper.
Transfer bean and potato puree to large buttered baking or gratin dish. Dot top with remaining 1 tablespoon butter. Bake 15 minutes, or until skin forms on top and puree is heated through. Serve right from baking dish.
Vegetable purees are not only satisfying ans tasty, they are also relatively unfussy. The can be pureed well ahead of time, then reheated in double boiler, or gratin?ed, as here.