1. Bring a pan of salted water to a boil and blanch the spinach for 30 seconds. Plunge the spinach into iced water to refresh, then drain and press out some of the water with your hands. Blitz the spinach in a food processor until it is smooth, then place it in a clean cheesecloth and press and squeeze out as much water as you can until the spinach purée is as dry as possible.
2. To make the Béchamel sauce, melt the butter in a small pan. Stir in the flour and cook for 3-4 minutes without letting it color, then whisk in the milk. Bring the sauce to a boil and whisk it until it is smooth. Season with the freshly grated nutmeg and some salt and pepper. Remove the pan from heat and whisk in the egg yolks, then cover the pan to prevent a skin from forming.
3. Preheat the oven to 390°F (200°C). Butter your ramekins or mould and coat the insides with grated Parmesan. Put 4 tbsp of Béchamel in a bowl, add 4 tbsp of the spinach and mix well. Whisk the egg whites in a separate bowl with a pinch of salt until stiff, then fold into the Béchamel and spinach mixture a little at a time until smooth. Do not over mix.
4. Pour the mixture into the ramekins or mould. Optionally, level off with a spatula. Run a knife or your thumb around the inside rim to ensure a straight rise. Place in the preheated oven and cook for 8-10 minutes for individual soufflés or 16-18 minutes for a bigger one. Serve immediately on its own or with a sauce such as tomato coulis.