Marinate the meat. Slice the pork tenderloin diagonally across the grain in 1/4 slices. In a bowl, combine the meat and 1 teaspoon cornstarch. Mix well until meat is coated. Add in 1 teaspoon oyster sauce, soy sauce and oil, then stir until incorporated. Cover and marinate for about 15 minutes.
Sweat the ampalaya (bitter melon or bitter gourd). Using a knife, split the vegetable into half, then scoop and scrape the core with a spoon. Slice to 1/4 inch thickness. To reduce bitterness, sprinkle with salt then toss to distribute salt. Let it sit for about 20-30 minutes, then rinse in cold water.
Heat oil in a large skillet on high heat. Toss the marinated pork, stirring occasionally, so it develops a sear on sides. Cook the meat to medium well doneness, about 4 minutes. Add onion, garlic, ginger and cook for about 1 minute. Pour 1/4 cup of water and let come to boil for 1 minute. Scrape off any bits from the pan as it adds flavor to the dish.
Add the ampalaya and lightly stir just enough to distribute evenly in the pan. Let simmer for about 5 minutes without stirring. Add the remaining water, oyster sauce, sugar and cornstarch slurry. Cook until the vegetable is done to tender-crisp and the sauce has thickened and turned into a glaze.