Season the 2 pounds chicken thighs on all sides with 2 teaspoons kosher salt and 1/2 teaspoon black pepper. Heat the 1/2 cup oil in a large stock pot or Dutch oven over medium-high heat. Once shimmering, place chicken pieces in the hot oil. Cook, undisturbed, until dark golden brown on one side, flip and repeat on other side. Transfer to a plate.
Lower heat to medium-low and add the 1/2 cup flour. Cook, stirring constantly, scraping the bottom of the pot with a wooden spoon to prevent sticking until flour is toasted and turns a dark caramel color. Be patient this will take about 10 minutes but it is worth it. This is where all the flavor is developed.
Add the 2 diced onions, 4 stalks chopped celery, and the 2 diced green bell peppers. Season with remaining teaspoon salt and 1/2 teaspoon pepper. Cook, coating in the roux until tender, about 8 minutes. Add 3/4 of the scallions and the clove of minced garlic and cook 1 minute more.
Stir in the 4 cups chicken stock (or broth), 3 tablespoons Worcestershire sauce, 1 teaspoon cayenne pepper, and 2 bay leaves. Scrape the bottom of the pot to get any stuck on bits. Finally add the 12 ounces of sliced sausage and return chicken to pot along with any accumulated juices. Bring to a boil over medium-high heat, then reduce heat to low and simmer, uncovered, until chicken is cooked through and stew has thickened, about 2 hours, stirring occasionally to prevent burning on the bottom.
Make the steamed rice while the gumbo is cooking. When it is nice and thick and the chicken is fall apart tender, taste the broth and add more salt and pepper if needed. You can remove the chicken pieces and shred the meat if you'd like or leave them whole.
Serve over steamed rice and garnish with remaining scallions and pass hot sauce at the table.