In a large saute pan, melt the butter over medium heat.
Add the onions and saute until translucent, about 15 minutes.
Stir in the flour and nutmeg and cook for about 2 minutes stirring constantly so the flour does not burn.
Pour in the cream and milk and cook until thickened.
Squeeze out as much liquid as possible from the spinach and add to the cream sauce.
Add 1 cup of Parmesan cheese, 1/2 cup of Gruyere and stir thoroughly.
Stir in the salt and pepper and remove from the heat.
Transfer the spinach mixture to a baking dish and sprinkle the remaining cheese over top evenly.
Bake for 20 minutes or until hot and bubbling.
Serve immediately.
Notes
Creamed Spinach Gratin can be assembled up to 2 days in advance and stored in the refrigerator tightly wrapped with plastic wrap.Uncover and bake as directed right before serving.