How to Make It
In a medium saucepan, combine the cream, milk and the 2 sticks of butter with the rosemary, sage and garlic and bring just to a simmer. Remove from the heat and let steep for 15 minutes, then discard the rosemary, sage and garlic.
Meanwhile, in a large pot, cover the potatoes with water and bring to a boil. Add a generous pinch of salt and simmer over moderate heat until tender, about 20 minutes. Drain well, then pass the potatoes through a ricer into the pot. Fold in the cream mixture and season generously with salt and pepper.
Light the broiler and position the rack 8 inches from the heat. Scrape the potatoes into a 12-inch round flameproof pan or baking dish (2 inches deep) and, using a spoon, decoratively swirl the top. Gently brush with melted butter. Broil for about 8 minutes, until the top is browned in spots. Serve hot.