Brush the inside of a large soufflé, deep casserole dish, or 6 individual ramekins with 1 1/2 tablespoons of softened butter. Sprinkle the buttered surface evenly with grated Parmesan cheese and put to one side while you prepare the filling.
Gently heat a large saucepan, add the chopped spinach and sauté until it wilts and the juices have evaporated—keep a close eye on this as the spinach can catch quite easily and burn.
Preheat the oven to 375 F.
In a separate medium saucepan, melt the remaining butter over medium heat and stir in the flour and salt with a wooden spoon. Cook, whisking constantly, for 30 seconds to cook out the flour.
Add the milk to the flour while whisking vigorously and cook for about 4 minutes, until the mixture thickens.
Add the chopped spinach to the flour mixture and continue cooking over medium heat for 1 minute. Season the mixture with black pepper and nutmeg.
Whisk 1/2 cup of the hot spinach into the egg yolks.
Then add the egg yolk mixture back into the hot spinach, stirring to completely incorporate. Do not overheat this mixture or you risk splitting the mixture, and if this happens, unfortunately, there is no way back.
Beat the egg whites on high speed until stiff peaks form in a scrupulously clean bowl (a greasy or dirty bowl will prevent the eggs from stiffening).
Stir one-third of the egg whites into the spinach.
Then fold the remaining egg whites into the mixture.
Spoon the mixture into the prepared dish(es) and bake for 30 minutes, until the soufflé is puffed up and cooked through.
Serve immediately. The soufflé is lovely on its own and with a fresh, green salad on the side and don’t forget a glass of wine.