Take defrosted chopped spinach and hand squeeze out excess liquid. Put dried out spinach in separate bowl. Put to the side.
Melt butter in pan and saute over medium heat. Add onions and saute for approximately 15 minutes or until translucent. Add flour and nutmeg and cook for additional 2 minutes. Add cream and milk and cook until it thickens. Add spinach and 1/2 cup of Parmesan cheese. Mix well. Season with salt and pepper to taste.
Transfer mixture to ungreased backing dish. Sprinkle remaining 1/2 cup parmesan and Gruyere cheese on top.