Season chicken with 1 ¼ teaspoons Cajun seasoning, ¾ teaspoon salt, and ½ teaspoon pepper.
Melt butter in a Dutch oven over medium heat. Working in batches, add chicken to the Dutch oven in a single layer and cook until golden brown and cooked through, reaching an internal temperature of 165 degrees F, about 4-5 minutes per side; set aside.
Add sausage, onion, bell pepper, and celery, and cook, stirring occasionally, until tender, about 5-7 minutes.
Stir in tomato paste, garlic, and 1 teaspoon of Cajun seasoning until fragrant, for about 1 minute.
Whisk in flour until lightly browned, about 1 minute.
Stir in chicken stock and diced tomatoes, scraping any browned bits from the bottom of the Dutch oven. Stir in orzo; season with salt and pepper to taste.
Bring to a boil; reduce heat and simmer, stirring occasionally, until pasta is cooked through, about 6 minutes. Return the chicken to the Dutch oven.
Serve immediately, garnished with green onions and parsley if desired.