Heat oil in a large, 6-quart stock pot over medium heat. Add the onions, peppers, celery and garlic - cook for 2 to 3 minutes until softened.
Add the cubed chicken and Cajun seasoning. Toss with the vegetables, stirring occasionally until chicken is cooked through, 5-7 minutes.
Stir in tomatoes, corn, carrot, chicken broth, barley and sliced sausage. Bring to a boil over high heat, then reduce to low and cook, covered, 25 to 30 minutes for pearled barley (35-45 minutes for hulled) or until the barley is tender.
Add spinach and cook another 3-5 minutes, uncovered, until the spinach is wilted.
Season to taste with salt and serve.
