Add celery, onion, bell peppers, garlic, diced tomatoes, chicken breasts, chicken stock, and Cajun seasoning to the slow cooker.
Cook on low 7-8 hours or high 3-4 hours.
30 minutes before the cook time is over, melt the butter in a pan over medium heat. Add flour slowly, whisking until combined and mixture is a light brown color. Add to the crockpot and stir.
Shred the chicken if needed. Garnish with green onions and serve!