Wash the spinach and drain in sieve. Don't dry the spinach--leave the water clinging to the leaves.
Heat a large skillet over medium-high heat. Add the wet spinach and cook, stirring occasionally, until wilted. Transfer the spinach to the sieve and press out as much liquid as possible.
Heat the butter in a large skillet over medium heat. Add the onion and cook, stirring, until soft, about 4 minutes. Stir in the spinach, cream, and nutmeg and season with salt and pepper. Cook the spinach, stirring occasionally until most of cream is absorbed, about 4 minutes.