In a bowl, combine pork, calamansi juice, soy sauce, and Knorr seasoning sauce. Mix well and marinate for 15 minutes.
Heat oil in a pan over high heat. Sear the pork slices until browned on both sides. Remove and set aside.
In the same pan, add the remaining marinade and oyster sauce. Bring to a simmer.
Return the pork to the pan and add the red onions.
Cover and simmer for 5 to 10 minutes, depending on the tenderness of the pork.
Uncover, season with salt and pepper, and add sugar if needed to balance the flavor.
Let the sauce reduce slightly until it reaches your desired consistency.
Garnish with reserved green onion ends and serve warm.