Spinach and Cheddar Souffle

Name: Spinach and Cheddar Souffle
Ingredients: 3 tablespoons unsalted butter, plus extra for greasing the dish; 1/4 cup finely grated Parmesan cheese, plus extra for sprinkling; 3 tablespoons all-purpose flour; 1 cup scalded milk; 1/4 teaspoon of nutmeg; Pinch of cayenne pepper; Kosher salt and freshly ground black pepper; 4 extra-large egg yolks, at room-temperature; 1/2 cup grated aged Cheddar cheese, lightly packed; 1 package frozen chopped spinach, defrosted and squeezed dry; 5 extra-large egg whites, at room temperature; 1/8 teaspoon cream of tartar