boil potatoes in a large pot about 20-25 min, until tender.
meanwhile, heat a cast iron skillet and thinly slice onions into half rounds. sweat the onions for a few minutes in the hot pan until they are soft and just starting to color. add salt, pepper and olive oil and continue cooking, stirring frequently, until nicely browned and caramelized. add the garlic toward the end of the onions' cooking time.
blanch kale in boiling water until wilted but still bright green. drain.
drain the potatoes and mash with the butter and milk. add the kale, onions and mustard and stir together well.
taste for salt & pepper. sprinkle with the nutmeg and top with a little extra butter if you like.