In a large skillet, saute onion in 1 tablespoon butter for 1 minute. Add apple; cook 2-3 minutes longer or until apple is golden brown. Remove from the heat; add walnuts, rosemary, and a 1/4 teaspoon each of salt and pepper.
Flatten chicken to 1/4 inch thickness; sprinkle with garlic powder and remaining salt and pepper. Place apple mixture and Brie on half of each chicken breast; fold chicken over. Secure with toothpicks if necessary.
In the same skillet, brown chicken in remaining butter. Stir in vinegar and 1/4 cup apple juice. Bring to boil. Reduce heat; cover and cook for 15-20 minutes or until a thermometer reads 170°F.
Remove chicken to a serving platter; discard toothpicks. Combine cornstarch and remaining apple juice; add to the pan. Bring to boil; cook and stir for 2 minutes or until thickened. Serve with chicken. Yield: 2 servings.