In a large bowl, combine the soy sauce, lemon juice, black pepper, and minced garlic. Add the thinly sliced beef and massage the marinade into the meat. Let it marinate for at least 15 minutes.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat.

Remove the beef slices from the marinade, reserving the liquid. Pan-fry the beef in batches for 1-2 minutes per side until browned. Remove the beef from the pan and set aside.

In the same pan, add the remaining tablespoon of oil and the sliced onions. Sauté for about 2 minutes until the layers separate and they are slightly softened but still crisp. Remove half of the onions and set aside for garnish.

Pour the reserved marinade and water into the pan with the remaining onions. Bring the mixture to a boil.

Reduce the heat to low, return the browned beef to the pan, and cover. Simmer for 10 to 15 minutes until the beef is tender and the sauce has reduced slightly. Stir in the sugar if you prefer a milder acidity.

Transfer the beef and sauce to a serving platter and top with the reserved crisp onion rings. Serve immediately with steamed rice.