How to Make It
Steam potatoes. Bring butter, milk, salt, and pepper to a simmer in a medium pot; pour over potatoes and mash.
Cook mustard greens and garlic in olive oil in a medium skillet over medium until tender, about 2 minutes. Stir greens, scallions, and cheese into potatoes. Transfer to a buttered 2-qt. baking dish. Bake at 400°F until browned.