Rinse turkey breast thoroughly with cold water; pat dry, and set aside.
Remove 1/2 cup Apple-Bourbon Marinade, and reserve for Apple-Bourbon Gravy. Pour remaining 2 cups Apple-Bourbon Marinade into a 2-gal. zip-top plastic freezer bag; add turkey breast. Seal bag, and chill at least 8 hours or up to 24 hours, turning occasionally.
Remove turkey breast from bag, discarding marinade.
Place flour in oven bag; shake to coat inside of bag. Place bag in a shallow roasting pan; place celery and sliced onion inside bag. Brush turkey breast with melted butter, and place on top of vegetables in bag. Close bag with nylon tie; cut 6 (1/2-inch) slits in top of bag.
Bake at 350° for 1 hour and 45 minutes or until a meat thermometer inserted into thickest portion of turkey breast registers 170°. Remove turkey breast from bag to a serving platter. Pour bag drippings through a wire-mesh strainer into a medium bowl, discarding solids in strainer and reserving drippings (about 2 cups) in bowl for Apple-Bourbon Gravy. Garnish, if desired, and serve with gravy.
Note: If doubling the recipe, simply place both oven bags side by side in the same roasting pan, and bake as directed.