Make pocket in each chicken breast by cutting horizontally from thickest side almost to opposite side. Spoon cheese into each pocket. Secure pockets with wooden skewers, if desired. Brush chicken pieces on both sides with 1 teaspoon olive oil. Sprinkle with salt and red pepper flakes.

In large nonstick skillet heat 2 teaspoons of remaining oil over medium heat. Add chicken. Cook uncovered for 12 minutes, turning once. Carefully add 1/4 cup chicken broth to skillet. Cook covered for 3-5 minutes more or until chicken is no longer pink. Remove chicken from skillet. Cover chicken to keep warm. If any liquid remains in skillet, gently boil until liquid evaporates.

Carefully add remaining 1 teaspoon oil to skillet. Stir in onion. Cook uncovered over medium-high heat until onion is soft. Add grapes. Continue cooking, stirring occasionally, about 3 minutes or until grapes just begin to split. Remove and set aside about 3/4 of grape-onion solids, leaving liquid and remaining grape-onion mixture in skillet.

Stir remaining 1/4 cup chicken broth, port wine and vinegar into grape mixture in skillet. Bring to boil, then reduce heat and simmer uncovered 3-5 minutes or until liquid portion is reduced to about 1/3 cup. Stir in reserved grape-onion solids.

To serve, remove wooden skewers from chicken, if using. Place chicken on serving plates and spoon grape mixture over chicken. Garnish with rosemary.