Coat a 1-quart souffle dish with cooking spray; sprinkle with 1 tablespoon breadcrumbs.
Heat a medium saucepan coated with cooking spray over medium-high heat. Add onion; saute 2 minutes or until tender. Remove from heat. Add flour, salt, peppers, and nutmeg, stirring well. Gradually add 1/2 cup milk, stirring with a whisk until well blended. Stir in 1/2 cup milk. Cook mixture over medium heat 2 minutes or until thick and bubbly, stirring constantly with a whisk. Remove from heat.
Place egg yolk in a bowl. Gradually add milk mixture to egg yolk, stirring constantly with a whisk. Return mixture to pan. Cook 1 minute or until thick. Remove from heat; stir in cheese and spinach. Cool 5 minutes.
Place egg whites and cream of tartar in a large bowl; beat with a mixer at high speed until stiff peaks form. Gently stir one-fourth of egg white mixture into milk mixture; gently fold in remaining egg white mixture and 2 tablespoons breadcrumbs. Spoon into prepared dish. Bake at 400° for 30 minutes or until souffle is puffy and set. Serve immediately.