Wash and clean Brussels sprouts, cut the stalk crosswise. Put some florets aside. Cook the rest in salted water for about ten minutes, quench and drain. Then cut into slices. Pluck the leaves from the rest of the Brussels sprouts and blanch them in salted water for one minute, rinse in cold water and drain.
Wash, peel and quarter the potatoes and cook in salted water for about 15 minutes. Drain, allow to steam off, mash coarsely with a potato masher while adding the butter. Fold in the Brussels sprouts slices, season with salt, pepper and freshly grated nutmeg. Serve garnished with the leaves.