In a heavy-based soup pot, heat 3 Tbs. of the oil over high heat until almost smoking. While the oil is heating, season the chicken generously with salt and pepper, dust it with 1/4 cup of the flour, and shake off the excess. Sear the chicken in the hot oil until golden brown, turning once to brown both sides, 4 minutes on each side. Remove the chicken and set aside.
Let the oil reheat for a minute and then reduce the heat to medium high and add the celery, onions, and peppers and cook until soft, 10 to 12 minutes, stirring to prevent scorching. Remove the vegetables and any liquid from the pan and set aside. Add 3/4 cup oil to the pot and let it heat up for a minute over medium heat. Slowly add the remaining flour to the pan and cook, stirring constantly, until the mixture is the color of chocolate, 10 to 20 minutes. Stir carefully, being sure to scrape the sides and bottom of the pot to prevent scorching. Pay attention not to burn the roux; if you do burn it, you’ll have to start over.
When the roux has reached a good mahogany brown, return the cooked vegetables to the pot, along with the garlic, cayenne, oregano, basil, thyme, and bay leaves. Stir to scrape the bottom of the pot and cook until well combined, 3 to 5 minutes. Season with salt and pepper.
Slowly add the broth while stirring until smoothly blended. Add the chicken and the sausage. Bring to a simmer and skim off excess fat. Simmer uncovered, skimming any foam or fat that rises to the surface, until the chicken meat is so tender that it falls easily from the bones, about 2 hours (begin checking earlier). Remove the bones from the pot and discard.
Return the soup to a boil and stir in the filé powder, stirring vigorously to avoid clumping, until the filé powder is dissolved. Taste for seasoning, adding salt and pepper as necessary. Serve the gumbo in bowls over the cooked white rice with chopped scallions and Tabasco to taste.