Cook the spinach with just the water that clings to it from cleaning. Cool. Squeeze as dry as possible with hands and chop finely. Saute the spinach and onion in the butter until soft. Transfer to a bowl. Stir in ricotta, Parmesan, and egg. Mix well. Season with nutmeg, salt and pepper. (You can mix in processor before adding the flour). Stir in flour. The mixture will be slightly sticky. Flour hands and shape mixture into small balls, no larger than 3/4 inch in diameter. Reflour hands whenever mixture begins to get sticky. Dry on a floured baking tray. To freeze: place in freezer on trays until firm. Transfer to plastic bags to store. Cream Sauce: Simmer butter and cream in a saucepan over medium heat until thickened, about 20 to 30 minutes. Bring a large pot of salted water to a boil. Drop in the gnocchi. Let the water return to a boil and cook gnocchi until they float to the top. Remove with a slotted spoon and place in a warmed bowl. Add Cream Sauce and Parmesan cheese. Gently toss to coat well. Serve immediately.