In a large bowl, combine the soy sauce, calamansi juice, minced garlic, and black pepper. Add the beef slices and mix well to coat. Let marinate for at least 20 minutes.

Heat the vegetable oil in a large skillet or wok over medium-high heat.

Remove the beef from the marinade (reserve the leftover liquid) and fry the slices in a single layer in batches. Cook for 1-2 minutes per side until browned and slightly crispy on the edges. Remove cooked beef and set aside.

In the same pan, sauté half of the red onion rings for about 1 minute until slightly softened but still crunchy.

Pour the reserved marinade, water, and brown sugar into the pan. Bring to a boil and simmer for 2-3 minutes to reduce the sauce slightly.

Toss the fried beef back into the pan to coat with the glaze.

Top with the remaining fresh onion rings, turn off the heat, and serve immediately with steamed rice.