Cajun Chicken Soup

Name: Cajun Chicken Soup
Ingredients: 2 tablespoons butter.; 1 ½ cups holy trinity (½ cup onion, ½ cup celery, ½ cup bell pepper, chopped).; 4 cloves garlic, minced.; 2 tablespoons Cajun seasoning, divided.; 2 cups chicken stock.; 1 (14.5-ounce) can diced tomatoes (do not drain).; 1 pound boneless, skinless chicken breasts, sliced thin.; 1 tablespoon neutral oil (e.g., avocado or canola).; 1 cup heavy cream or half-and-half, at room temperature.; Chopped green onions (optional, for garnish).