This should do it:
1 lb russet (baking) potatoes
⅓ cup heavy cream
5 teaspoons unsalted butter
¼ teaspoon minced garlic
1 large egg
½ teaspoons salt
⅛ teaspoon white pepper
¼ teaspoon freshly grated nutmeg
Put oven rack in middle position and preheat oven to 400°F.
Prick each potato a few times, then bake in a shallow baking pan until tender, about 1 hour.
When potatoes are just cool enough to handle, halve and scoop out flesh. Force flesh through ricer or food mill into a bowl.
Heat cream with butter and garlic in a 1-quart heavy saucepan over moderately low heat until hot, then stir into potatoes. Beat in egg with a fork until blended, then stir in salt, white pepper, and nutmeg. Spoon mixture into a buttered 1-quart au gratin dish or Pyrex pie dish. Bake until top is puffed and golden, 20 to 25 minutes.