Microwave chopped onion in a small microwave-safe bowl at HIGH 2 minutes or until tender. Set aside.
Sprinkle chicken evenly with 1 tsp. Creole seasoning. Sauté chicken in a lightly greased Dutch oven over medium-high heat 5 minutes or until lightly browned. Remove chicken using a slotted spoon.
Whisk together 4 cups water, broth, and roux mix. Pour mixture into Dutch oven, and stir to loosen particles from bottom. Bring to a boil over medium-high heat, whisking until smooth. Reduce heat to medium-low, and simmer 5 minutes. Stir in onion, chicken, and remaining 1/2 tsp. Creole seasoning. Cover and cook, stirring occasionally, 20 minutes. Stir in green onions.
Stack tortillas; cut into 2- x 1-inch strips. Add strips, 1 at a time, making a single layer over top of mixture in Dutch oven; gently submerge strips. Return mixture to a low boil; repeat with remaining strips. Cook 3 to 4 more minutes, stirring gently after 2 minutes. Garnish, if desired.
Note: For testing purposes only, we used Tony Chachere's Creole Instant Roux Mix.