Combine tomatoes, green bell pepper, onion, and celery in a large skillet over medium heat; cook and stir until fragrant, about 10 minutes. Reduce heat and cook until green bell pepper and onion are soft, about 5 minutes more. Stir in sugar, salt, oregano, basil, and pepper.
Whisk water and cornstarch together in a small bowl. Stir into the skillet. Simmer tomato mixture until thickened, about 5 minutes.