1) Cut and steam the potatoes until tender. Return to the still-hot pot and add 55g butter, milk, salt and 1/4 teaspoon pepper. Mash together.
2) Melt the remaining butter in a large pot over medium heat. Add shallots and leeks. Saute until they begin to soften. Add the kale and toss until soft and wilted but still bright green, about 3 minutes. Add the cabbage and toss until tender-crisp, about 8 minutes. Sprinkle with nutmeg, salt and the remaining pepper.
3) Mix the cabbage and mashed potatoes together well. Sprinkle with the green onion slices before serving.