Slice sirloin into thin strips. Make sure you slice the meat against the grain for the most tender bite.
In a bowl, combine the baking soda, soy sauce, oyster sauce, black pepper, and cornstarch, and mix until combined. Add the sliced beef sirloin. Coat well and marinate for a minimum of 15 minutes.
Slice the bitter melon into half and remove the seeds. Slice in moon shape and transfer into a bowl with water and sprinkle a half teaspoon of salt to remove some bitterness. Soak for 10-15 minutes. Drain the water.
Heat 2 tablespoons of oil in a skillet over medium heat. Cook the beef with the marinade. Allow the beef to cook slightly seared. This takes about 3-5 minutes. Take it out and set it aside on a plate.
In the same pan, add a tablespoon of oil. Saute garlic and onion until translucent. Add the tomatoes and cook them until soft.
Add the ampalaya, spread across the pan, and cook for 3 minutes undisturbed.
Return the seared beef to the skillet. Stir to combine and cook for a few minutes or until the beef is heated thoroughly and the bittermelon is crisp and tender. Taste and adjust the seasonings according to your taste. Enjoy!