Fold excess parchment over potatoes and cover pan with aluminum foil. Bake at 350°F until potatoes are tender, about 2 hours. Remove foil from pan, leaving parchment in place.

In a large saucepan, melt 2 tablespoons of the butter. Add the onion and cook over moderate heat, stirring, until lightly browned, 7 minutes. Add the rice and cook, stirring, until well coated ...