In a medium saucepan cook chopped cabbage and thinly sliced onion in a small amount of boiling water for 5 minutes, or until tender-crisp. Drain.
In a large saucepan combine milk, 2 Tbsp butter, and salt; bring to a boil. Remove from the heat and stir in instant potato flakes. Let stand for minute. Fluff with a fork then add egg, 1/2 cup shredded Swiss cheese, tarragon, thyme, and pepper; mix well.
In a 9-inch square baking pan, layer half the potato mixture, corned beef, the cabbage, and 2 cups shredded Swiss cheese; repeat the layers. Combine 2 Tbsp melted butter and bread crumbs; sprinkle over the cabbage mixture.
Bake, uncovered, at 350° (175°C) for 20 minutes.
Servings: 6