Heat 1 Tbsp. oil in large skillet or wok. Stir-fry onions and mushrooms 3 minutes over medium-high heat. Add celery and cook 1 more minute. Add remaining vegetables and cook 2 minutes or until green pepper is tender crisp. Transfer vegetables to warm bowl. Add remaining 1 Tbsp oil to skillet. Stir-fry meat in oil about 2 minutes or until meat loses its pink color.
Blend cornstarch and water. Stir into meat. Cook and stir until thickened. Return vegetables to skillet; stir gently and serve.