Put the spinach into a pan without additional water.
Cover and cook gently for 3 minutes, shaking the pan frequently.
Cool slightly and put into the processor bowl.
Add half the butter and the cream and process to make a puree .
Put the puree into a greased ovenproof dish, and put the flounder fillets on top.
Discard the crusts and make the bread into fine crumbs .
Grate the cheese .
Mix the crumbs and cheese and season lightly.
Sprinkle on top of the fish and dot with flakes of the remaining butter.
Bake at 400Â°F for 20 minutes.