Heat pan on medium heat. Add 1 tbsp cooking oil and saute the garlic until golden.
Place the sliced beef and stir occasionally until juices dry up and all sides are seared.
Add the soy sauce, vinegar, brown sugar (optional), black pepper, and water.
Mix everything well, then bring the liquid to a gentle boil over medium heat. Do not stir immediately after adding the vinegar. Allow it to boil uncovered for about 1 minute first.
Stir the beef, reduce the heat to medium-low, and cook uncovered for 8β12 minutes, or until the beef becomes tender and most of the liquid has evaporated. Stir occasionally so the meat cooks evenly.
As soon as only a small amount of sauce remains, add the remaining cooking oil.
Increase the heat to medium-high and cook the beef for another 3β5 minutes, stirring and turning the pieces continuously, until lightly caramelized around the edges.
Taste and adjust with a little more soy sauce, vinegar, or sugar as needed.
Remove from the heat and serve hot with garlic rice, fried eggs, or sliced tomatoes.