Cook and stir tomatoes, bell pepper, onion, and celery in a large skillet over medium heat until fragrant, about 10 minutes. Reduce the heat and continue to cook until bell pepper and onion are soft, about 5 minutes. 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.