Place oil in a 5qt dutch oven over medium-high heat. Place onions into hot oil and sauté for 3-5 minutes or until tender. Add chicken broth, enchilada sauce, cumin, garlic seasoning, green chilies, and tomatoes. Stir and bring to a boil. Reduce heat to low then add sour cream, cheese, and chicken. Stir and let simmer for 15-20 minutes or until cheese is melted through.