Place a large saucepot over high heat; add half of the spinach to the dry hot pot and 1/2 cup water. Place lid on top, allow spinach to steam for about 2-3 minutes. Remove the spinach to a sheet tray fitted with a rack and repeat the process with remaining spinach. Once all the spinach is wilted, use a clean kitchen towel to ring out the excess liquid.
Using same pot the spinach was cooked in, melt butter and sauté the garlic 1-2 minutes. Whisk in flour and grate in nutmeg, season with salt and ground black pepper. Cook 1 minute, add half-n-half and bring to a bubble. Simmer at a low bubble to thicken, about 10-15 minutes.
Meanwhile, separate spinach with your fingertips and add it to the thickened sauce. Fold in cheese and serve.