1. In a 1-quart microwavable casserole, grease by spraying nonstick cooking spray.
2. In a small mixing bowl, blend 3 tablespoons water, soy sauce and cornstarch with a wired whisk; stir until the cornstarch is dissolved.
3. Add celery, mushrooms, onion, and 1 tablespoon water; cover and cook on High for 4 minutes until celery is tender; stir once in between cooking.
4. Stir in the cornstarch mixture; cover and cook on High for 1 1/2 minutes until the mixture thickens.