Place a baking sheet in oven. Preheat oven to 425 degrees.
Coat 4 (6-ounce) ramekins with cooking spray and sprinkle evenly with breadcrumbs, tilting and turning dishes to coat sides completely.
Heat a large skillet over medium-high heat. Coat pan with cooking spray. Add spinach; cook 2 minutes or until spinach wilts, tossing constantly. Place spinach in a colander; let stand 5 minutes. Squeeze excess liquid from spinach. Coarsely chop.
Combine milk and flour, salt, nutmeg and pepper in a small saucepan over medium-high heat, stirring with a whisk until smooth. Cook 2 minutes or until mixture is thick and bubbly, stirring constantly. Spoon mixture into a large bowl and let stand 10 minutes. Stir in spinach, cheese and egg yolks.
Combine egg whites and cream of tartar in a large bowl, and let stand at room temperature 15 minutes. Beat with a mixer at high speed until medium peaks form (do not overbeat). Gently stir one-quarter of the egg whites into spinach mixture, and gently fold in remaining egg whites.