Sauté Vegetables: In a large pot, melt 2 tablespoons of butter over medium-high heat. Add the 1 ½ cups of the holy trinity (onion, celery, bell pepper) and sauté until tender, approximately 5 minutes. Once tender, add 4 cloves of minced garlic and sauté for an additional 30 seconds until fragrant.
Simmer the Soup Base: Pour in 2 cups of chicken stock along with 1 can of diced tomatoes. Stir well and bring the mixture to a low boil. After reaching a boil, reduce the heat and let it simmer.
Cook the Chicken: While the soup base simmers, season 1 pound of thinly sliced boneless, skinless chicken breasts with the remaining 1 tablespoon of Cajun seasoning. In a separate pan, heat 1 tablespoon of neutral oil. Cook the chicken for 3-5 minutes per side until golden and cooked through. Remove from heat, let cool, and then chop into bite-sized pieces.
Combine and Serve: Lower the heat on the soup base. Stir in 1 cup of heavy cream until thoroughly combined. Next, add the cooked chicken to the pot. Allow the soup to simmer for a few more minutes to blend all the flavors together. For presentation, garnish with chopped green onions if desired, and serve warm!