Cut tops off peppers and remove the core and seeds. Dice the pepper tops and set aside.
Place the peppers on a baking sheet and bake for 20 minutes.
Meanwhile, season chicken on all sides with 1 tablespoon Cajun seasoning. Heat 1 tablespoon oil in a large frying pan set over medium heat. Add the chicken and cook for 4 to 6 minutes, until golden and nearly cooked through. Remove and set aside. Add sausage to the now-empty pan and cook, stirring occasionally, until lightly browned, 1 to 2 minutes. Add celery, onion, garlic and the reserved diced pepper and cook until the onions are translucent, 3 to 5 minutes. Add remaining 1 tablespoon Cajun seasoning, tomatoes, tomato paste, stock and rice, and add the chicken back into the pan. Stir to combine and bring to a boil then reduce the heat to a simmer and cook, stirring occasionally, until the chicken has cooked through and the rice has softened, 5 to 10 minutes. Remove from heat and let it stand, covered, until all the liquid is absorbed, about 10 minutes. Divide the chicken mixture among the peppers, spooning into each one and mounding it on top, if necessary. Bake until everything is heated through and the peppers are fully cooked and soft, about 20 minutes.