Pour the 12 cups water into a large pot, add 1 tablespoon salt, and bring to a boil over medium-high heat.
Meanwhile, swirl the spinach in a separate large bowl of cold water, changing the water several times, to remove any sand or grit. Drain the spinach in a colander. Fill the large bowl halfway with ice water.
Add the spinach to the boiling water and cook for 2 minutes.
Using tongs or a slotted spoon, transfer the spinach to the ice water for just a minute or so (this halts the cooking and preserves the vibrant color). Drain the spinach in a colander, squeezing out any excess water. (Seriously. You gotta squeeze that spinach till not a drop of water is left.)
Wipe out the pot and place it over medium heat. Add the butter and heat until it melts. Add the onion and 1 teaspoon salt and cook until the onions are softened but not browned, about 4 minutes. Add the spinach, cream, mascarpone or additional cream, Parmigiano-Reggiano, and nutmeg.
Cook, stirring, for 2 to 3 minutes, until the mixture is warm throughout. Season with salt and 6 grinds white pepper. Serve as soon as you can.