Instructions

1. Separate 5 large eggs: Place all 5 egg whites in a stand mixer (or large bowl if using an electric hand mixer or whisking by hand). Place 4 of the egg yolks in a small bowl; reserve the remaining egg yolk for another use. Let the eggs sit until room temperature, at least 20 minutes. Meanwhile, arrange a rack in the middle of the oven, remove any racks above it, and heat the oven to 400ºF.
2. Coat a 6- to 8-cup soufflé dish or 2-quart baking dish (like an 8-inch square that’s 2 inches high) generously with butter. Finely grate 1 ounce Parmesan cheese on the small holes of a box grater (about 1/4 cup plus 2 tablespoons, or 1/4 cup pre-grated Parmesan cheese). Sprinkle 2 tablespoons of the Parmesan evenly onto the bottom and the sides of the baking dish.
3. Grate 1 1/2 ounces Gruyère cheese on the large holes of the box grater (about 1/2 packed cup). Finely dice 1 small shallot (about 1/3 cup).
4. If using frozen spinach, place 10 ounces in a microwave-safe bowl and microwave in 1-minute bursts until defrosted, stirring between each burst, 2 1/2 to 3 minutes total. If using baby spinach, blanch 12 ounces in a large pot of boiling water until wilted, then drain and rinse with cold water.
5. Transfer the spinach to a strainer. Using your hands, squeeze all the excess moisture from the spinach. If using fresh spinach, very finely chop it.
6. Melt 3 tablespoons unsalted butter in a small saucepan over medium heat. Add the shallot and cook until softened, about 3 minutes. Add 3 tablespoons all-purpose flour and cook, whisking constantly, until the floury taste is cooked out, about 1 minute.
7. While whisking constantly, add 1 cup whole or 2% milk and whisk until the flour is dissolved. Add 1 1/4 teaspoons kosher salt, 1/4 teaspoon black pepper, 1/8 teaspoon freshly grated nutmeg, and a pinch of cayenne pepper. Bring to a simmer, stirring constantly and scraping in the corners of the pan, 2 to 4 minutes.
8. Remove the saucepan from the heat. Whisk in the egg yolks one at a time, making sure each egg yolk is fully incorporated before adding the next. Add the Gruyère, spinach, and remaining Parmesan, and stir until combined with no big clumps of spinach. Scrape into a large bowl.
9. Add 1/8 teaspoon cream of tartar to the egg whites. Beat with the whisk attachment on medium speed until firm, glossy peaks form, 3 to 4 minutes.
10. Transfer about 1/4 of the egg whites into the spinach mixture and stir until combined. Add the remaining egg whites and gently fold with a rubber spatula until just combined with no large streaks of egg white. Transfer the mixture to the soufflé or baking dish and smooth out the top. Pick up the baking dish and gently tap the bottom onto the counter to help the soufflé settle.
11. Place the baking dish in the oven. Immediately reduce the oven temperature to 375ºF. Bake until puffed, there are no visible wet streaks of egg, and the top is evenly golden-brown, 40 to 45 minutes for a soufflé dish or about 30 minutes for a regular baking dish (do not open the oven door before time is up!). Serve immediately, as it will deflate quickly.