In a nonstick saucepan, heat 1 tablespoon butter over medium heat. Add onion, and cook until softened, about 8 minutes. Place milk and flour in a jar with a tight-fitting lid. Shake vigorously to combine; pour into saucepan, and cook over medium-high heat, stirring constantly, until mixture thickens and bubbles, about 7 minutes. Stir in pizza sauce, and remove from heat.