Melt butter in a saucepan. Add flour and stir until crumbly. Gradually add milk and cook over medium-low heat until thick, stirring constantly, about 15 minutes. Remove from heat and add finely chopped chicken and onion. Stir in seasonings. Blend together and chill for 45 minutes. Preheat oven to 450?F. Shape chicken mixture into six cones or cylinders, about 1/3 cup each. Roll in bread crumbs, then egg whites, then bread crumbs again. Place on baking sheet and spray each croquette with non-stick vegetable coating. Bake 15 minutes, then reduce oven temperature to 350?F and continue baking for 20 minutes or until outsides are browned.