Preheat your oven to 375°F (190°C).

In a medium saucepan, melt the butter over medium heat and add the chopped spinach. Cook until wilted, about 2-3 minutes.

Add the flour to the spinach mixture and stir for 1-2 minutes to create a roux.

Slowly whisk in the milk, stirring continuously until the mixture thickens, about 3-5 minutes.

Separate the egg yolks from the whites, placing the yolks in a large bowl and the whites in a separate mixing bowl.

Stir the cooked spinach mixture, salt, pepper, nutmeg, and cheese into the egg yolks.

Using a clean whisk, whip the egg whites until stiff peaks form.

Gently fold the egg whites into the spinach mixture until just combined.

Pour the mixture into a greased baking dish and bake for 30 minutes or until puffed and golden brown.