In a large skillet, heat oil over moderate high heat. Add onions and cook 3 minutes. add remaining vegetables, toss to coat with oil and cover skillet. Cook about 8 minutes until the vegetables are barely tender but still retain their crunch. Add soy and stir well. Arrange cheese over vegetables, cover and remove from heat. Let stand for 3 minutes until cheese is melted. Sprinkle with sesame seeds and serve immediately.