Method:

1. Place the sliced steak in a large bowl. Sprinkle the cornstarch over the beef slices and mix to coat. Whisk the soy sauce and sugar together in a small bowl until the sugar dissolves; pour over the beef slices. Mix and marinate in the refrigerator for at least 2 hours or overnight.

2. Heat 3 tablespoons olive oil in a large skillet over medium heat, cook and stir the onion and garlic in the hot oil until tender, 5 to 7 minutes. Stir the oyster sauce into the onion and garlic, season with salt and pepper. Add the snow peas, green peas, carrot, celery, and red bell pepper. Cook and stir until the vegetables are slightly tender, yet still crisp, 7 to 10 minutes. Remove the skillet from the heat.

3. Heat 2 tablespoons oil in a separate skillet. Remove the beef from the marinade, shaking off any excess moisture, discard the marinade. Fry the beef slices in the hot oil to your desired degree of doneness, 3 to 5 minutes per side for medium. Add the cooked beef to the vegetable mixture and mix well. Serve hot.