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

In a large skillet, melt 2 ounces of butter over medium heat and sauté the cabbage for about 5 minutes until softened.

In a mixing bowl, combine the boiled and mashed russet potatoes and turnips with the sautéed cabbage, remaining butter, kosher salt, and black pepper, mixing until smooth.

Pour the mixture into the prepared baking dish and spread evenly across the bottom.

Sprinkle the grated sharp cheddar cheese over the top of the mixture.

Cover with aluminum foil and bake for 30 minutes, then remove the foil and bake for an additional 5 minutes until the cheese is golden.

Let the casserole cool for 5 minutes, cut into squares, and serve.