Combine beef with all the marinade ingredients. Mix well. Marinade for 30 minutes.
1 ½ lb. beef sirloin, 5 tablespoons cooking oil, 3 tablespoons oyster sauce, 1/4 cup soy sauce, 2 teaspoons Sesame oil, 2 ½ teaspoons cornstarch, ¾ teaspoon baking soda, 6 cloves garlic, 1 thumb ginger, ½ teaspoon ground black pepper
Heat 2 tablespoons of cooking oil in a pan. Add the marinated beef slices. Stir fry for 2 minutes. Transfer to a plate. Set aside.
Heat the remaining cooking oil on the same pan used to cook the beef. Saute the onion until it softens.
1 onion
Add the bitter gourd. Continue to stir-fry for 1 minute. (see notes below)
2 pieces bitter gourd
Add the beef back into the pan. Pour water. Cover and let boil.
3/4 cup water
Remove the cover and simmer while stirring occasionally until the sauce thickens. (see notes below)
Transfer to a serving bowl. Serve!