Melt the butter in a large skillet and sauté the onion until tender but not browned.
Add the spinach, cover, and cook for five minutes. Turn onto a board and chop again. Place spinach in a bowl and add the salt, pepper, nutmeg and one-halfcup of the cheese; mix well.
Pour the hot cream over the eggs while beating. Gradually beat into the spinach mixture and pour all into the pie shell. Sprinkle with remaining cheese. Bake for thirty minutes, or until set. Serve warm.