2 tablespoons unsalted butter, plus softened butter for the ramekins
Finely grated Parmesan, for the ramekins
3 tablespoons all-purpose flour
2/3 cup milk, warmed
1/4 cup frozen spinach, thawed, drained and finely chopped
4 large egg yolks
1/4 teaspoon cayenne pepper (or paprika)
1/4 teaspoon freshly grated nutmeg
Pinch kosher salt
Freshly ground black or white pepper
6 large egg whites
2 ounces grated Gruyere
Preheat the oven to 425°F. Using a pastry brush, coat six 6-ounce ramekins with softened butter. Lightly coat with Parmesan and tap out any excess.
In a medium saute pan over medium heat, melt the butter, then add the flour and cook for 2 minutes, stirring constantly so it doesn’t darken. Pour in the milk and whisk vigorously until thickened, about 3 minutes.
Remove from the heat. Add the spinach and stir to combine. Add the yolks, one at a time, whisking well after each addition. Whisk in the cayenne, nutmeg, salt, and some pepper.
In a stand mixer fitted with the whip attachment, beat the egg whites on medium-high speed until just stiff with glossy peaks.
Add one-third of the whipped whites into the spinach mixture and stir well. Add another third of the egg whites, this time gently fold them in. Add the remaining egg whites and fold until just combined, but no white streaks of egg are visible. Fold in the Gruyere.
Spoon the soufflé into the prepared ramekins to the tops of the rims and smooth the top with an offset spatula. Place the ramekins on a rimmed baking sheet. This can be done hours before baking and stored in the refrigerator.
When ready to bake, run a knife around the top edge of each ramekin to ensure the souffle will rise in the oven. Drop the oven temperature to 400°F and bake the soufflés on the convection setting for about 25 minutes or until puffed up, with golden brown tops. (If you don’t have convection, continue baking at 425°F.) Serve immediately.