Preheat oven to 375°.
Combine first 5 ingredients in a small bowl. Cut a horizontal slit through thickest portion of each chicken breast half to form a pocket. Divide cream cheese mixture evenly among pockets. Place bread in a food processor; pulse 10 times or until coarse crumbs measure 1 cup. Brush mustard evenly over tops of chicken. Top with breadcrumbs, pressing down slightly. Place chicken, crumb side up, on a baking sheet coated with cooking spray.
Bake at 375° for 20 minutes or until done.