Preheat oven to 425 degrees F. Place a baking sheet in oven. Coat 4 (6-ounce) ramekins with cooking spray and sprinkle with breadcrumbs, coating sides completely.

Heat a large skillet over medium-high heat; coat with cooking spray. Add spinach; cook 2 minutes or until spinach wilts, tossing constantly. Place spinach in a colander and squeeze out any excess liquid from spinach. Coarsely chop.

In a small saucepan over medium heat, combine milk, flour, salt, nutmeg and black pepper, whisking until smooth. Cook for 2 minutes or until mixture is thick and bubbly. Transfer mixture to a large bowl and let stand 10 minutes. Stir in spinach, cheese and egg yolks.

In the bowl of an electric mixer fitted with the whisk attachment, combine egg whites and cream of tartar in a large bowl, and let stand at room temperature for 15 minutes. Whisk on high speed until medium peaks form. Gently stir one-third of egg whites into spinach mixture, and then fold in remaining egg whites. Spoon mixture into prepared dishes. Sharply tap dishes 2 or 3 times on counter to level. Place dishes on preheated baking sheet; return baking sheet to oven. Immediately reduce oven temperature to 350 degrees F and bake for 20 minutes or until puffy and golden brown. Serve immediately.