Heat 1/2 teaspoon oil in a large nonstick skillet over medium heat. Add sliced onion; sauté 30 minutes or until golden brown. Add sliced garlic; sauté 5 minutes. Stir in 1/3 cup wine; cook 5 minutes or until liquid almost evaporates. Spoon onion mixture into a bowl; let cool. Stir in Brie, salt, and pepper.
Cut a horizontal slit through the thickest portion of each chicken breast half to form a pocket. Stuff about 1 1/2 tablespoons of the onion mixture into each pocket.
Heat 1/2 teaspoon oil in skillet over medium-high heat. Add chicken; sauté 6 minutes on each side or until chicken is done. Remove chicken from skillet. Set aside; keep warm.
Add 1/3 cup wine, minced onion, sage, and minced garlic to skillet. Cook over medium-high heat for 2 minutes. Stir in broth. Bring to a boil, and cook 7 minutes or until reduced to 3/4 cup. Return chicken to skillet; cover and simmer 2 minutes or until thoroughly heated. Serve sauce with chicken. Garnish with fresh sage, if desired.