Prep the Soufflé Dish
1-2 T of unsalted butter for greasing the dish
1/4 cup finely grated Parmesan cheese (save extra for sprinkling on top)

Butter the inside of one 6-to-8-cup soufflé dish (6-1/2 to 7-1/2 inches in diameter x 3-1/2 inches deep) and sprinkle evenly with Parmesan.

Roux
3 T (42 g) unsalted butter
3 T (23 g) all-purpose flour
1 cup (244 g) COLD whole milk (I’ve found that adding cold milk to a hot roux = no lumps)
1/4 tsp freshly ground nutmeg
1/2 tsp coarse kosher salt
1/4 tsp freshly ground black pepper
4 extra-large egg yolks, (76 g) at room temperature

1/2 cup (57 g) grated aged Cheddar cheese, lightly packed
1 10 oz (283 g) package frozen chopped spinach, defrosted and squeezed dry

Melt the butter in a small saucepan over low heat. With a wooden spoon, stir in the flour and cook, stirring constantly, for 2 minutes.

Whisk in the cold milk, nutmeg, salt, and black pepper. Cook over low heat, whisking constantly, until smooth and thick (a minute or two).

Off the heat, but while still hot, mix in the egg yolks one at a time. Then stir in the Cheddar, the Parmesan, and the spinach, and transfer to a large mixing bowl. (You can make this part of the recipe up to two hours ahead. Keep the mixture covered at room temperature and then proceed with the meringue addition just before baking.)

Preheat the oven to 400°F (204°C).

Meringue
5 extra-large egg whites (225 g), at room temperature*
1/2 tsp cream of tartar

* Important!! An easy trick to warm them straight from the fridge is to set them in a bowl of warm water for five minutes before cracking them open.

Put the egg whites, cream of tartar, and a pinch of salt in the bowl of an electric mixer fitted with the whisk attachment. Beat on low speed for one minute, on medium speed for one minute, then finally on high speed until firm, glossy peaks are formed.

Whisk one-quarter of the egg whites into the cheese sauce to lighten, and then fold in the rest. Pour into the prepared soufflé dish, then smooth the top. Draw a large circle on top with the spatula and place in the middle of the oven. Turn the temperature down to 375°F (191°C).

Bake for 30 to 35 minutes (don’t peek!) until puffed and brown. Serve immediately.

Adapted from a recipe by Ina Garten, Barefoot Contessa