Spinach Soufflé

Name: Spinach Soufflé
Prep time: 30 min
Cook time: 52 min
Total time: 1 h 22 min
Cuisine: French
Ingredients: 5 large eggs; 1 ounce Parmesan cheese; 1 1/2 ounces Gruyère cheese; 1 small shallot (about 2 ounces); 10 ounces frozen chopped spinach, or 12 ounces baby spinach; 3 tablespoons unsalted butter, plus more for the baking dish; 3 tablespoons all-purpose flour; 1 cup whole or 2% milk; 1 teaspoon kosher salt; 1/4 teaspoon freshly ground black pepper; 1/8 teaspoon freshly grated nutmeg; Pinch cayenne pepper; 1/8 teaspoon cream of tartar