Preheat your oven to 350°F (175°C) and grease a 9x9 inch baking dish with 1 ounce of unsalted butter.

Melt 2 ounces of butter in a large skillet over medium heat and sauté the thinly sliced cabbage for about 5 minutes until slightly softened.

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

Transfer the mixture into the greased baking dish, spreading it evenly, and top with 1 ounce of grated sharp cheddar cheese.

Cover with aluminum foil and bake for 30 minutes. Remove the foil and bake for an additional 5 minutes to brown the cheese.

Allow the Rumbledethumps to sit for about 5 minutes before serving.