Remove the top rack in the oven and preheat to 450°. In a large pot of boiling salted water, cook the spinach until wilted, about 1 minute. Drain and let cool. Squeeze the spinach dry and coarsely chop it.
In a blender, mix the eggs, milk, flour, 1 1/2 teaspoons of salt, 1/8 teaspoon of pepper and the nutmeg until smooth.
In a 12-inch cast-iron skillet, melt the butter. Add the scallions and cook over moderate heat until tender, about 3 minutes. Add the spinach and cook until heated through, about 1 minute; season with salt and pepper. Increase the heat to high and cook for 1 minute without stirring. Pour in the batter, sprinkle with the Parmesan and transfer the skillet to the oven. Bake for 25 minutes or until golden. Cut into wedges and serve hot or warm.
MAKE AHEAD The Dutch baby can be baked up to 4 hours ahead; reheat in the skillet in a 425° oven for 5 minutes.