This is an old favorite of mine
1 pkg. frozen chopped spinach, defrosted and squeezed dry
1/2 C. butter
1/4 C. flour
2/3 C. milk
2 egg yolks beaten well
2 egg whites beaten stiff with 1/8th tsp cream of tartar
(this helps stableize the whites and keep their volume
you could use a bit of lemon juice instead)
salt and freshly ground black pepper to taste
1/2 C. grated cheese
Heat the butter in a skillet and stir in the flour whisk in the milk and cook over low heat until thickened. Stir in the spinach and then the yolks and cheese and combine well. Fold in the whites and pour into a 1 1/2 qt. souffle or baking dish. Bake at 350 F 35-40 minutes.
I spray the souffle dish with cooking spray and then dust the inside with freshly grated parmesan cheese. It adds great flavor and gives the souffle something to hang onto as it climbs. You can also sprinkle a little on the top when you take it out of the oven.