Sprinkle 2 teaspoons of Poultry Magic on the chicken pieces, patting it in with your hands. Combine 1 teaspoon of Poultry Magic with the flour in a paper or plastic bag.
Heat about ¼-inch oil in a large heavy skillet to about 350º. Dredge the chicken pieces in the seasoned flour, shaking off excess (reserve leftover flour). Cook in the hot oil until golden brown, about 2 minutes per side. Do not crowd. (Adjust heat as necessary to maintain oil's temperature at about 350º. If drippings start to burn, discard the oil and drippings and use fresh oil.) Drain chicken on paper towels.
Combine the onions and celery in a small bowl and set aside.
Pour off all but ½ cup of hot oil from the skillet, leaving as much sediment in the pan as possible. Return skillet to high heat and let remaining oil heat until it starts to smoke, about 2 minutes, scraping loose any sediment that may be stuck to pan bottom. With a long-handled metal whisk gradually whisk in the reserved flour until smooth. Continue cooking, whisking constantly, until the roux turns dark red-brown, about 2 to 3 minutes (being careful not to let it scorch or splash on your skin). Immediately stir in the reserved vegetable mixture and remove from heat. Continue stirring (change to a wooden spoon if necessary) until the roux stops darkening in color, about 3 minutes. Set aside.
In a 5½ quart saucepan or large Dutch oven, bring the stock to a boil. Add the roux to the stock by spoonfuls, whisking until roux dissolves between each addition. Stir in the remaining 1 tablespoon Poultry Magic and the gumbo filé (if desired). Add the chicken to the pot and cook about 5 minutes stirring occasionally. Reduce heat to low and continue cooking until chicken is tender and sauce is reduced to about 4 cups, about 50 to 60 minutes, stirring occasionally. Serve immediately over rice or with mashed potatoes, allowing 2 pieces of chicken and a generous portion of sauce for each person.