Add chicken and continue cooking until chicken is brown, about 3 to 5 minutes, stirring frequently and scraping pan bottom well. Stir in the seasoning mix and 1/2 cup each of the onions, celery, and bell peppers and the garlic. Cook until vegetables start to get tender, about 5 to 8 minutes, stirring fairly constantly and scraping pan bottom as needed. Stir in the tomato sauce and cook about 1 minute, stirring often. Stir in the remaining 1/2 cup each of the onions, celery, and bell peppers and the tomatoes.