Preheat oven to 400 degrees F. Line a large rimmed baking sheet with parchment paper or foil.

Cut tops off peppers and carefully remove the core and seeds, taking care not to split the skin. Dice the pepper tops and set aside. Place the peppers on the prepared baking sheet; bake for 20 minutes. Remove from oven and let cool. Discard any accumulated liquid in the bottom of the peppers.

Meanwhile, season chicken on all sides with 1 Tbsp. Cajun seasoning. Heat 1 Tbsp. oil in a large skillet over medium heat. Add half of the chicken and cook, turning to brown all sides, 4 to 6 minutes. Transfer the chicken to a medium bowl with a slotted spoon. Repeat with the remaining 1 Tbsp. olive oil and the remaining chicken.

Add sausage to the now-empty skillet and cook, stirring occasionally, until lightly browned, 1 to 2 minutes. Add celery, onion, and the reserved diced pepper; cook, stirring often, until the onions are translucent, 3 to 5 minutes. Add garlic, the remaining 1 Tbsp. Cajun seasoning, and salt; cook, stirring, for 30 seconds. Add tomatoes and tomato paste; stir to combine, scraping any brown bits off the bottom of the pan. Add broth, cauliflower rice, and the chicken with any accumulated juices; stir to combine. Bring to a boil. Reduce heat to maintain 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 stir. Let stand, covered, until all liquid is absorbed, about 10 minutes.

Divide the chicken mixture among the peppers, spooning about 1 1/4 cups into each one and mounding it on top, if necessary. Bake until heated through, about 20 minutes.