Boil 1 pound quartered, peeled Yukon gold potatoes in salted water until tender. Reserve 1/4 cup cooking water, then drain. Saute 2 cups shredded cabbage in butter, 2 minutes. Add the reserved cooking water, cover and cook 3 minutes. Add the potatoes, 4 sliced scallions, 1/2 cup half-and-half, 2 tablespoons butter and a pinch of nutmeg. Mash until smooth. Season with salt and pepper.