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1
Pour water to a depth of 1 inch into a large pot and bring to a boil.
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2
Put the potatoes into a collapsible steamer basket and set the basket over the boiling water.
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3
(The water should not touch the bottom of the steamer basket.)
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4
Cover and steam until the potatoes are tender when pierced with a small knife, about 15 minutes.
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5
Transfer the potatoes to a bowl.
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6
Empty the pot and wipe dry Return the potatoes to the still-hot pot.
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7
Add 1/4 cup of the butter, the milk, 3/4 teaspoon kosher salt, and 1/4 teaspoon pepper and mash well with a potato masher.
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8
Melt the remaining 1/4 cup butter in another large pot over medium heat.
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9
Add the leek and shallots and saute until the vegetables begin to soften, about 5 minutes.
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10
Add the kale and toss just until wilted but still bright green, about 3 minutes.
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11
Add the napa cabbage and toss until tender-crisp, about 8 minutes.
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12
Sprinkle with the mace, 1/4 teaspoon kosher salt, and `/4 teaspoon pepper.
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13
Cover and cook until the flavors blend, about 1 minute.
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14
Stir the cabbage mixture into the potatoes.
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15
Reheat the potatoes over low heat, stirring often, about 5 minutes.
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16
Transfer to a bowl, sprinkle with the green onion, and serve.
