Steam the potatoes in a vegetable steamer for about 20 minutes, until tender.

Set the potatoes aside to cool slightly.

Bring a large pot of salted water to a boil.

Stem the greens and blanch the leaves in the boiling water until tender, about 3 minutes.

Drain the greens, and cool under cold running water.

Drain the greens well, wringing out the excess water with your hands.

Finely chop the greens.

When the potatoes are just cool enough to handle but still warm, peel them.

Mash the potatoes coarsely in a large bowl.

Add the greens and scallions to the mashed potatoes.

Stir in the hot milk to the desired consistency.

Add the butter and creme fraiche, and season with salt and pepper.

To make creme fraiche, whisk together 1 part buttermilk with 8 parts non-ultra-pasteurized (and preferably non-homogenized) heavy cream in a bowl.

Cover the bowl with a cloth or plastic wrap, and leave it out on the counter for 24 to 48 hours, until it thickens.

Whisk the creme fraiche again before storing, tightly covered, in the refrigerator, where it will keep for several weeks; it gets thicker and more delicious as it ages.