Begin with the meatballs (bola-bola). Soak bread slices (tinapay) in milk (gatas) until completely soft. Tear into a paste to create a panade. In a large bowl, gently combine ground beef (karneng giniling), the bread paste, grated onions (kinayod na sibuyas), beaten egg (binating itlog), salt (asin), and pepper (paminta). Mix just until combined - overmixing will make tough meatballs.
Form the mixture into 1-inch meatballs using a small ice cream scoop (panukat) or wet hands. Place them on a lined tray and freeze for 15 minutes until firm.
Meanwhile, prepare the sauce (sarsa). Combine pineapple juice, rice vinegar (suka), ketchup, and brown sugar (pulang asukal) in a bowl. In a separate small bowl, mix cornstarch with water (tubig) until smooth.
Heat oil (mantika) in a large pan over medium-high heat. Briefly sauté bell peppers (bell pepper) until slightly tender but still crisp; remove and set aside. In the same pan, brown the chilled meatballs in batches until golden on all sides. Don't overcrowd the pan. Remove and drain.
Lower heat to medium and cook sliced onions (sibuyas) until soft. Pour in the sauce mixture and bring to a boil. Add meatballs back to the pan and simmer for 8-10 minutes. Stir in the cornstarch mixture to thicken. Finally, add the bell peppers and pineapple chunks (pinya), cooking just 1-2 minutes more until heated through.
Let rest for 5 minutes before serving with steaming white rice (kanin).