1. Heat oil in a large soup pot over medium heat. Add onion and sauté until soft. Add garlic, zucchini, and coleslaw mix. Cook until vegetables are soft, about 10 minutes.
2. Add water, tomato sauce, ketchup, vinegar, brown sugar, and seasonings. Bring to a boil. Stir in cornstarch.
3. Let simmer for one to one and a half hours.