Start by slicing the flank steak thinly against the grain to keep it tender. In a mixing bowl, combine soy sauce, hoisin sauce, oyster sauce, cornstarch, and sesame oil. Add the beef slices and toss well until fully coated, then let it marinate for about 15 minutes so the flavors soak in deeply.
Wash the cucumbers thoroughly and use a vegetable peeler to shave them into long, thin ribbons. If the center is too seedy, stop peeling once you reach it. Place the ribbons on paper towels to remove excess moisture, which helps keep the final dish crisp and fresh.
Heat vegetable oil in a large skillet or wok over medium-high heat until shimmering. Add the marinated beef in a single layer and cook quickly without overcrowding the pan. Let it sear for 1–2 minutes before flipping so it develops a slight caramelized crust.
Add minced garlic and ginger to the pan once the beef is nearly cooked. Stir for about 30 seconds until fragrant, then mix in rice vinegar and brown sugar. Let everything simmer briefly so the sauce thickens and coats the beef beautifully.
Arrange the cucumber ribbons neatly on a serving platter to create a cool, refreshing base. Spoon the hot, glossy beef over the top or serve it on the side for wrapping. Finish with sliced green onions and a sprinkle of sesame seeds for garnish.