2 1/2 pounds russet potatoes, peeled
8 tablespoons unsalted butter, plus more for serving
1/4 medium head green cabbage, cored and thinly shredded
1 cup milk
1/3 cup heavy cream
4 scallions, green parts only, finely chopped
Kosher salt, to taste
freshly ground black pepper, to taste
Place potatoes in a 6-qt. saucepan and cover with water by 1"; bring to a boil over high heat, and cook until tender, about 30 minutes.
Drain, quarter, and set aside. Return pan to medium-high heat, and add butter; when melted, add cabbage, and cook, stirring, until wilted, about 5 minutes.
Add milk, cream, and scallions, and bring to a boil; add potatoes, and using a potato masher, mash and stir potatoes until smooth and evenly incorporated. Season with salt and pepper, and transfer to a bowl.
Serve hot with large pats of butter on top.