The potatoes can be made a day in advance. To reheat them, place them in a bowl, cover the bowl with foil, and set it over a double boiler.
Source: Martha Stewart Living, November 1999
3 pounds russet potatoes, peeled and quartered
Salt and freshly ground black pepper
3/4 cup milk
8 tablespoons (1 stick) unsalted butter, cut into pieces
2 tablespoons olive oil
1 large yellow onion, halved and quartered
3 1/2 ounces Roquefort cheese, crumbled (1/2 cup)
Place potatoes in a large saucepan of cold salted water. Bring water to a boil. Reduce to a simmer, and cook until potatoes are fork-tender, about 20 minutes. Warm milk in a small saucepan. Drain potatoes, and return to the pan. Add warm milk, butter, salt, and pepper.
Mash the mixture with a potato masher until all the liquid is incorporated and the butter has melted. Transfer mixture to a metal mixing bowl. Cover with aluminum foil; set over double boiler to keep warm.
Meanwhile, heat olive oil in a large skillet on medium heat. Add onions, and saute stirring often, until golden brown, 10 to 12 minutes.
Adjust the seasoning of potatoes with salt and pepper, if desired. Transfer the potatoes to a serving dish. Spoon the onions over the potatoes, and crumble Roquefort cheese over the top. Serve immediately.