Ingredients
- 4 tablespoons unsalted butter, plus more for baking dish
- ½ cup grated Parmesan, divided
- 1 medium shallot, finely chopped
- 8 ounces wild mushrooms (such as oyster, shiitake, or a mix), stems removed and chopped (about 4 cups)
- 1 10-oz. package frozen chopped spinach, thawed and squeezed dry
- 4 ounces Gruyere, grated (1 cup)
- ¼ cup all-purpose flour
- 1½ cups whole milk
- Pinch nutmeg
- 6 large eggs, separated
How to Make It
- Preheat oven to 400°F with oven rack in lowest third of oven. Generously butter a 2-quart soufflé or baking dish and coat with ¼ cup Parmesan.
- Melt butter in medium saucepan over medium heat. Add shallot and mushrooms and cook, stirring often, until mushrooms begin to soften, about 5 minutes. Add flour and cook, stirring with a wooden spoon, until nutty smelling, about 3 minutes. Gradually whisk in milk in 4 additions. Bring to a gentle simmer and cook, whisking often, until thickened and mixture coats a spoon, 4 to 5 minutes. Remove from heat and stir in the spinach, ½ teaspoon kosher salt, several grinds black pepper, a pinch of nutmeg, and gruyere. Whisk in the egg yolks.
- Beat egg whites with an electric mixer on low speed. When they begin to foam, increase speed to high and beat until stiff peaks form, about 3 minutes. Gently fold ¼ of the whites into mushroom mixture, then fold mixture into remaining whites. Quickly transfer to prepared dish and top with remaining ¼ cup Parmesan.
- Transfer to oven on lower rack and immediately reduce temperature to 375°F. Bake until puffed and golden, 25 to 30 minutes.
