Put 1 1/2 pounds unpeeled all-purpose potatoes, cut into 1-inch cubes, in a medium saucepan and cover with water. Add 1 teaspoon salt and boil over moderate heat until tender, about 15 minutes. Drain, return to the pan and keep warm.
Melt 4 tablespoons unsalted butter in a skillet. Add 1 minced onion and cook over moderately low heat, stirring, until well browned, about 20 minutes.
Mash the potatoes with 1/4 cup hot milk and 2 tablespoons butter. Stir in the onions and season with salt and pepper.