Cut ampalaya lengthwise into halves. With a spoon, remove seeds and scrape off inner pith. Sliced thinly and place in a bowl of cold water until needed. Drain well when ready to use.
In a bowl, combine beef, 1/2 of the oyster sauce, 1/2 of the soy sauce, sesame oil and salt and pepper to taste. Marinate for about 10 minutes.
In a wide skillet over high heat, heat 1 tablespoon of the oil. Add beef and spread across pan. Allow to sear for about 1 to 2 minutes or until lightly browned and then turn to brown evenly. Remove from pan and set aside.
In the same pan, add the remaining 1 tablespoon of oil. Add onions and garlic and cook, stirring regularly, until limp. Add ampalaya and gently stir to combine. Cook for about 2 to 3 minutes or until tender yet crisp. Return beef to pan.
In a small bowl, combine water, the remaining soy sauce and oyster sauce, sugar and cornstarch. Stir until sugar and cornstarch are dissolved. Add to pan and continue to cook, stirring regularly, for about 1 to 2 minutes or until beef is heated through and sauce slightly thickens. Season with salt and pepper to taste. Serve hot.