In a large bowl, combine the soy sauce, calamansi juice, black pepper, and half of the minced garlic. Add the beef slices, mix well to coat, and marinate for at least 30 minutes in the refrigerator.

Heat 1 tablespoon of vegetable oil in a large skillet over medium-high heat. Remove beef from the marinade (reserve the liquid) and pan-fry the slices for 1-2 minutes per side until browned. Remove beef and set aside.

In the same skillet, add the remaining 1 tablespoon of oil. Add the red onion rings and sauté until they soften and become slightly translucent, about 3 minutes.

Add the sliced mushrooms and the remaining minced garlic to the pan. Sauté for another 4-5 minutes until the mushrooms are browned and tender.

Pour the reserved marinade and the water into the pan. Bring the mixture to a simmer and gently scrape the bottom of the pan to release flavor.

Stir in the brown sugar to balance the acidity. Return the cooked beef and its juices to the pan.

Simmer for 2-3 minutes until the sauce reduces slightly and the beef is heated through. Serve hot over steamed white rice.