In Large non stick skillet, cook and stir onion in 1 teaspoon oil over medium heat for 10 minutes. Stir in brown sugar and vinegar, cook 5-10 minutes more or till tender and golden brown. Remove, keep warm.
In same skillet brown chicken in remaining oil on both sides, add broth, stirring to loosen browned bits from pan. Transfer to 11 x 7 inch baking dish coated with spray; top with onion mixture.
Place bagette slice over each chicken breast, sprinkle with cheese. cover and bake at 350 degrees for 15 minutes. Unover and bake 3 to 5 minutes longer or till meat thermometer reads 170 degrees.