Fill a large pot with water, add a generous pinch of salt, and bring it to a rolling boil. Carefully add the cubed potatoes to the boiling water. Cook for about 15-20 minutes until the potatoes are fork-tender. Once cooked, drain them thoroughly and set aside to cool slightly.
In a large skillet, melt 2 tablespoons of unsalted butter over medium heat. Add the chopped onions to the skillet, cooking until they are golden and translucent, around 5 minutes. Next, stir in the shredded cabbage and chopped kale, continuing to sauté for an additional 5-7 minutes until the greens are wilted and tender.
In a mixing bowl, take the drained potatoes and mash them using a potato masher until smooth with no lumps remaining. Mix in the remaining 2 tablespoons of butter until well combined. Then, fold in the sautéed onion, cabbage, and kale mixture. Add the grated cheddar cheese and season generously with salt and freshly ground pepper. Sprinkle in a pinch of nutmeg if you desire a hint of warmth, and mix until the ingredients are evenly combined.
Preheat your oven to 375°F (190°C). Brush a baking dish with a little butter or non-stick spray to grease it. Transfer the hearty Rumbledethumps mixture into the prepared dish, spreading it evenly. For an extra cheesy crust, sprinkle a handful of grated cheddar cheese over the top.
Place the baking dish in the preheated oven and bake for 25-30 minutes, or until the top is beautifully golden and bubbly, creating a delightful crust.
Once baked, carefully remove the dish from the oven and allow it to rest for a few minutes. Just before serving, sprinkle the freshly chopped chives on top for a pop of color and fresh flavor.
Notes
Serve with crusty bread and a side salad for a complete meal.