Wrap tortillas in aluminum foil; bake at 375° for 10 minutes or until thoroughly heated.
While tortillas bake, combine chicken and taco seasoning in a heavy-duty, zip-top plastic bag. Seal bag; shake well.
Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add chicken and onion; saute 6 minutes or until chicken is done.
Spoon chicken mixture evenly onto tortillas; top evenly with lettuce, tomato, and sour cream. Roll up tortillas, and wrap in unbleached parchment paper, if desired. Serve immediately.