This lighter take on creamed spinach can be an airy additionto the dinner table or a brunch buffet.
Preheat oven to 375 degrees. Butter a 9-by-1 1/2-inch round baking dish; set aside. Cover potato with water in a small saucepan. Add a pinch of salt. Bring to a boil; reduce to a simmer, and cook, uncovered, until tender, about 12 minutes. Drain. Pass through a ricer into a bowl; set aside.
Wash spinach, and drain, but do not dry. Transfer to a large skillet. Cover, and cook over medium-high heat, stirring once, until wilted, about 3 minutes. Drain. Let cool slightly; squeeze out any excess liquid. Coarsely chop.
Stir together the potato, spinach, ricotta, egg yolk, nutmeg, 1/2 teaspoon salt, and the pepper in a large bowl; set aside.
Put egg whites, cream of tartar, and a pinch of salt into the bowl of an electric mixer fitted with the whisk attachment; beat on high speed until stiff glossy peaks form, about 3 minutes. (Do not overbeat.)
Whisk one-third of the egg-white mixture into spinach mixture. Gently fold in remaining egg-white mixture. Transfer to prepared baking dish. Bake until slightly puffed and set in center, about 20 minutes. Serve immediately.