Preheat your oven to 350°F (175°C) and grease a 9x9 inch baking dish with unsalted butter.

In a skillet over medium heat, melt 2 ounces of butter. Add the sliced green cabbage and sauté for about 5 minutes until it softens.

In a large mixing bowl, combine the boiled and mashed russet potatoes, mashed turnips, and sautéed cabbage. Add 1 ounce of butter, kosher salt, and black pepper, mixing until fully incorporated.

Spread the mashed mixture evenly into the prepared baking dish and sprinkle a generous layer of grated sharp cheddar cheese on top.

Cover the baking dish with foil and bake for 30 minutes. Remove the foil and bake for an additional 5 minutes or until the top is lightly browned.

Once baked, let the casserole sit for about 5 minutes before serving.