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, ½ of the oyster sauce, ½ of the soy sauce, sesame oil, and 3 teaspoons of the cornstarch. Marinate for about 15 to 20 minutes. Drain well, reserving marinade.
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 softened.
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, the remaining 1 teaspoon cornstarch, and sugar. 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 and the sauce thickens.
Season with salt and pepper to taste. Serve hot.
Notes
Keep stirring to a minimum. Cook on high heat to sear the meat properly and prevent the bitter gourd from overcooking in the steam.