In a large bowl, combine the soy sauce, calamansi or lemon juice, and ground black pepper. Add the thinly sliced beef sirloin, ensuring each piece is well coated with the marinade. Cover and let it rest for at least one hour in the refrigerator — overnight marination yields the most flavorful results.

Warm the cooking oil in a wide skillet over medium heat. Add half of the onion rings and sauté until they turn translucent and slightly caramelized. Remove the onions and set them aside for garnishing later.

Drain the beef from the marinade, reserving the liquid. Using the same pan, sear the beef slices in batches for about one minute per side, or until they are browned but still tender. Transfer the cooked beef to a plate and set aside.

If necessary, add a touch more oil to the pan. Sauté the minced garlic and the remaining raw onions until fragrant and the onions begin to soften. Pour in the reserved marinade along with one cup of water. Stir well and bring the mixture to a gentle boil.

Return the seared beef to the pan, cover, and lower the heat to maintain a simmer. Cook until the meat becomes tender and the sauce slightly thickens, adding more water as needed to prevent drying out. Adjust the seasoning with salt and freshly ground black pepper according to taste.

Top the dish with the previously pan-fried onions before serving. Transfer to a platter and enjoy the rich, savory-sour flavors of Bistec served hot with a side of steamed rice.