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

Melt 2 ounces of unsalted butter in a medium skillet over medium heat. Add sliced green cabbage and sauté for about 5 minutes until softened.

In a large mixing bowl, combine boiled and mashed Russet potatoes and turnips, sautéed cabbage, remaining butter, kosher salt, and black pepper. Mix until smooth.

Spread the mixture evenly into the greased baking dish and flatten the top with a spatula.

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

Cover the dish with foil and bake for about 30 minutes. Remove foil and bake for an additional 5 minutes until cheese is bubbly and lightly browned.

Let the Rumbledethumps rest for 5 minutes before serving.