Scottish rumbledethumps is a dish from the border lands of southern Scotland and northern England

Cook the Potatoes: Place the diced potatoes in a large pot of salted water. Bring to a boil and cook until the potatoes are tender, about 10-15 minutes.
Drain the potatoes and return them to the pot. Add half of the butter (1/4 cup) and mash the potatoes until smooth. Season with salt and pepper to taste. Set aside.
Sauté the Onions and Cabbage: In a large skillet, melt the remaining butter (1/4 cup) over medium heat. Add the diced onions and cook until they begin to soften, about 5 minutes.
Add the shredded cabbage to the skillet and continue to cook, stirring occasionally, until the cabbage is tender and slightly wilted, about 10 minutes. Season with salt and pepper to taste.
Combine Ingredients: Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the mashed potatoes with the sautéed cabbage and onions. Stir until well combined.
Bake the Rumbledethumps: Transfer the potato and cabbage mixture to a greased baking dish, spreading it out evenly. Sprinkle the shredded cheddar cheese over the top.
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted, bubbly, and golden brown.
Serve: Remove the Rumbledethumps from the oven and let it cool slightly before serving. Enjoy it on its own or as a side dish.