In a large skillet, saute onion in 1 Tbsp. butter about 1 minute. Add apple; cook 2-3 minutes longer or until apple is golden brown. Remove from heat; add walnuts, rosemary & a dash of salt & pepper.
Flatten chicken breasts to 1/4″ thickness; sprinkle with garlic powder & remaining salt & pepper. Place apple mixture & Brie on half of each chicken breast; fold over. Secure with toothpicks if necessary.
In same skillet, brown chicken in remaining butter. Stir in cider vinegar & 1/4 cup apple juice. Bring to a boil. Reduce heat; cover & cook for 15-20 minutes or until meat thermometer reads 170 F.
Remove chicken to serving platter; discard toothpicks. Combine cornstarch & remaining apple juice; add to the pan. Bring to a boil; cook & stir for 2 minutes or until thickened. Serve with chicken.
Recipe Notes
This recipe is another one of my favorites I acquired from the tasteofhome.com site.