Preheat oven to 375°F/190C. Grease 1-1/2- or 2-quart soufflé dish; set aside.
Bring 1 quart salted water in 2-quart saucepan to a boil over high heat. Add spinach. Return to a boil and cook 2 to 3 minutes or until spinach is crisp-tender. Drain spinach and immediately plunge into cold water. Drain again; let stand until cool enough to handle. Squeeze spinach to remove excess moisture. Finely chop spinach.
Melt butter in large saucepan over medium heat. Add onion; cook and stir 2 to 3 minutes. Stir in flour, salt, nutmeg and pepper. Gradually stir in milk. Cook and stir until mixture comes to a boil and thickens. Remove from heat.
Stir egg yolks into saucepan until well blended. Add spinach and cheese; mix well.
Beat egg whites in clean large bowl with electric mixer at high speed until stiff peaks form. Fold egg whites into spinach mixture until egg whites are evenly incorporated. Pour into prepared dish.
Bake 35 to 40 minutes or until puffed and wooden skewer inserted in center comes out clean. Garnish, if desired. Serve immediately.