Preheat oven to 425F. Lightly oil a 3-quart baking dish.
Place potatoes in a large heavy pot. Add salted water to cover and bring to a boil. Reduce heat to medium and cook until tender, 10 to 20 minutes.
Meanwhile, drop cabbage into a pot of boiling salted water and cook, stirring occasionally, until tender, about 6 minutes. Drain and set aside.
Drain potatoes and return to pot. Place over low heat and shake, uncovered, for about 30 seconds to evaporate excess moisture.
Remoe from heat and mash potatoes with an electric mixer or a potato masher. Add cream cheese and 1 teaspoon salt. Mash untiil smooth. Fold in scallions and reserved cabbage. Adjust seasoning with salt and pepper. Spread mixture into prepared baking dish and top with cheese. (The casserole will keep, covered, in the refrigerator for up to 2 days.)
Bake casserole, uncovered, for 30 to 50 minutes before serving.