Bring 1 cup water to a boil in a wide large pot over high heat, then cook spinach, tossing with tongs, until completely wilted, about 5 minutes. Drain in a colander, pressing with back of a spoon to extract as much liquid as possible.

Heat butter in same pot over medium heat until foam subsides, then stir in nutmeg, 1/2 teaspoon salt, 1/4 teaspoon pepper, and spinach. Cook, tossing with tongs, until just heated through.

Cooks' note: Spinach can be washed and dried 1 day ahead and chilled, wrapped in paper towels, in a sealed bag.