In medium skillet combine oil, cornstarch, sugar, onion, prepared mustard, salt, garlic salt, and pepper.
Add water and vinegar; cook and stir till mixture thickens and bubbles.
Add zucchini and celery.
Cover and cook till vegetables are crisp tender, 7 to 8 minutes; stir occasionally.
Add tomatoes; cover and cook the vegetable mixture till heated through, 2 to 3 minutes more.