Heat the butter in a large (12-inch) skillet over medium heat. Add the shallot, coriander, and red pepper flakes, and cook, stirring occasionally, until the shallot begins to soften but does not brown, 2 to 3 minutes. Increase the heat to medium high, and begin adding the spinach, a large handful at a time, tossing with tongs, until all the spinach is in the skillet. Cook, tossing frequently, until the spinach is wilted and bright green, about 2 minutes. If a lot of water remains, cook on high heat, until the leaves are coated with butter but not soupy, another 1 to 2 minutes. Season to taste with salt, pepper, and nutmeg.