Cut and steam the potatoes until tender. Return to the still hot pot and add 1/4 cup butter, milk, kosher salt and 1/4 teaspoons pepper. Mash together.
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.
Mix the cabbage and mashed potatoes together well.
Sprinkle with the green onion slices before serving.