Preheat oven to 425˚. Line a baking sheet with parchment paper.
Bring a large pot of water to boil and add spinach. Stir once, and then drain spinach in a colander. Press down on spinach with a large spoon to extract as much water as possible.
Cut garlic clove in half. Rub inside of a 6-cup, straight-sided casserole dish with the cut side of the garlic. Coat inside dish with ½ tablespoon butter. Put about 1 ounce grated cheese in dish and shake until shreds adhere to bottom and sides.
Put remaining 2 tablespoons butter in a medium saucepan over medium-high heat, stirring with a whisk until melted. Add flour and cook, whisking constantly, until mixture turns light brown. Add milk in a stream, whisking briskly. As soon as mixture thickens, about 30 seconds, remove from heat and whisk in egg yolks, followed by 4 ounces cheese. As soon as cheese melts, stir in spinach, salt, and nutmeg.
In a large clean bowl, beat egg whites with a large whisk or mixer until stiff but not dry, about 4 minutes. Stir one-fourth of whites into yolk mixture; then gently fold in remaining whites thoroughly. Pour mixture into prepared dish and smooth top. Sprinkle with remaining 1 ounce grated cheese. Run a clean knife around inside edge of dish to a depth of about 1 inch (this helps the soufflé rise).
Bake for 30–35 minutes, until soufflé has risen and browned nicely. Serve at once.
Amount Per Serving
Calories 245Calories from Fat 153
% Daily Value*
Total Fat 17g26%
Saturated Fat 11g55%
Polyunsaturated Fat 1g
Monounsaturated Fat 5g
Total Carbohydrates 7g2%
Dietary Fiber 1g4%
* Percent Daily Values are based on a 2000 calorie diet.