Melt two tablespoons of the butter in a large skillet. Add the onion and saute until soft but not brown. Finely chop spinach and add to skillet, cooking over medium heat until the spinach has wilted and most of the liquid in the pan has evaporated.

In a separate sauce pan melt the remaining three tablespoons of butter. Add the flour and cook for a minute or two. Add the milk and continue cooking over medium heat, stirring constantly, until the sauce comes to a boil, is thickened and smooth. Remove from heat. Add two-thirds cup of the sauce to the spinach. Add parsley, salt and pepper to taste to the spinach.

Preheat oven to 350 degrees. Lightly butter a shallow baking pan. Season fish with salt and pepper, arrange in the baking pan and spread the spinach over the fillets.

Add the cream to the remaining sauce in the saucepan and heat it through. Season with salt and pepper, spread it over the spinach and fish, sprinkle with grated cheese and bake 35 to 40 minutes.