In a medium bowl, combine the thinly sliced beef sirloin, soy sauce, fresh calamansi juice, minced garlic, and cracked black pepper. Toss the mixture well to ensure every piece of beef is coated, then cover and let it marinate at room temperature for 15 minutes.

Heat one tablespoon of vegetable oil in a large skillet over medium-high heat. Add the onion rings and sauté them for 3 to 4 minutes until they are slightly softened but still retain a bit of crunch, then transfer them to a plate and set aside.

In the same skillet, heat the remaining tablespoon of oil over high heat. Sear the marinated beef slices in a single layer for 1 to 2 minutes per side until they are nicely browned, working in batches if necessary to avoid crowding the pan.

Pour the remaining marinade from the bowl and a half-cup of water into the skillet with the seared beef. Bring the mixture to a gentle boil, then lower the heat to medium-low and stir in the brown sugar to balance the tangy and salty flavors.

Simmer uncovered for 8 to 10 minutes until the beef is beautifully tender and the savory sauce has reduced slightly. Top the beef with the reserved sautéed onion rings, let them warm through for 1 minute, and serve hot over steamed white jasmine rice.