Golden Colcannon Pie

Steam cabbage in a steamer set over boiling water, covered, until just tender, 15 to 25 minutes. Remove lid, and when cool enough to handle, finely chop leaves. Preheat oven to 375 °F with rack in middle. While cabbage is steaming, peel and quarter potatoes. Generously cover potatoes with cold water in a 3- to 4-quart saucepan and add 1 teaspoon salt. Bring to a boil, then reduce heat, and briskly simmer, partially covered, until tender, 25 to 30 minutes. Drain potatoes well and mash in a large bowl with 3 tablespoons butter. Stir in cabbage, scallions, and salt and pepper to taste. Cool to warm and stir in egg. Melt remaining 2 tablespoons butter. Brush pie plate with some of butter. Spread potato mixture evenly in plate and brush with remaining butter. Bake until golden, about 35 minutes. Let stand 10 minutes before serving.