1/2 cup unsalted butter, at room temperature or 1/4 cup olive oil
1/4 cup whole milk or milk alternative
1 teaspoon kosher salt
1/2 teaspoon ground pepper, to taste
1 large leek, including green parts, halved lengthwise and rinsed well, then thinly sliced
4 large shallots, thinly sliced, or 2 medium onions
1 bunch kale, rinsed well and de-stemmed, then coarsely chopped
1 head napa cabbage or other cabbage, cored and coarsely chopped
1/8 teaspoon freshly grated nutmeg
2/3 cup chopped green onion tops
In the just emptied large pot, warm 1/4 cup butter or 1/8 cup oil, the milk, 3/4 teaspoon salt, and 1/4 teaspoon pepper. Add the potatoes and mash well with a potato masher.
In a sauté pan, melt the remaining 1/4 cup butter or 1/8 cup oil. Add the leek and shallots and sauté until soft, approximately 5 minutes. Add the kale and toss until bright green, approximately 3 minutes. Add the cabbage and toss until tender-crisp, about 8 minutes. Sprinkle with the nutmeg, remaining 1/4 teaspoon salt, and remaining 1/4 teaspoon pepper. Cover and cook for 1 minute. Stir the cabbage mixture into the mashed potatoes.
Reheat the potatoes over low heat, stirring often, about 5 minutes. Transfer to a bowl, sprinkle with the green onion, and serve.