1 cup whole milk
2 tablespoons unsalted butter
2 tablespoons all-purpose flour
4 large egg yolks
1 tablespoon Dijon mustard
1/2 cup plus 1 Tbsp. grated Gruyère
1 10-oz. package frozen chopped spinach, thawed and squeezed dry
Salt and pepper
6 large egg whites
- Preheat oven to 400°F. Butter a 2-quart soufflé dish.
- Bring milk to a simmer in a pan over medium-high heat. Melt butter in a separate heavy pan over medium-high heat. Whisk in flour; cook for about 1 minute. (Do not let it brown.) Whisk in hot milk. Cook until thickened and smooth, about 1 minute. Reduce heat to low; whisk in yolks, one at a time, mixing well. Remove from heat and whisk in mustard, 1/2 cup cheese and spinach. Season with salt and pepper.
- Using an electric mixer and a clean, dry bowl and beaters, beat whites until stiff peaks form. Stir 1/4 of whites into spinach mixture. Fold in remaining whites. Pour into soufflé dish.
- Sprinkle remaining 1 Tbsp. Gruyère over top. Reduce oven temperature to 375°F; bake until lightly browned on top and firm around edges but still a bit jiggly in the middle, 30 to 35 minutes. Serve immediately, mixing firm edges with softer center in every serving.