Marinate beef for 1 hour. Drain and reserve marinade.
Fry beef in 1/4 cup oil until light brown. Retain 2 Tbsp. oil in pan.
In the same pan, saute onion until almost tender.
Add soy sauce, 1/8 teaspoon pepper, calamansi juice and marinade.
Simmer for 2 minutes. Add fried beef and Del Monte fresh cut pineapple tidbits.
Simmer again for a few minutes and serve.