SAVORY GREEN SAUCE
2 garlic cloves, peeled
1/2 cup yogurt or sour cream
1 teaspoon freshly grated horseradish root, or use prepared horseradish
2 cups mixed cooking greens and herbs, such as a combination of fresh spinach, young kale, or mustard greens
Position rack in the center of the oven and preheat to 450°F.
Place the potatoes on a baking sheet. Bake until the potatoes are tender, about 1 hour. Cut each potato in half lengthwise, and scoop out the flesh into a bowl, leaving potato-skin shells. Mash the potato flesh with the butter and milk, and season with the salt and pepper. Refill the shells and, if necessary, return to the oven to keep warm.
In the meantime, make the sauce. Fit a food processor with the metal blade. With the machine running, drop the garlic through the feed tube to mince the garlic. Add the yogurt and horseradish and pulse to combine. Add the greens and process until the greens are pureed. Serve the potatoes hot, with the sauce passed on the side.