Prepare the Pickled Onion: Thinly slice a red onion and place it in a jar. In a pot, combine equal parts vinegar and water with a pinch of sugar and salt; bring to a boil, then pour over the onions. Cool and refrigerate for at least 30 minutes.

Marinate the Beef: In a bowl, combine the beef cubes with oyster sauce, soy sauce, fish sauce, sugar, black pepper, and grated garlic. Mix well, cover with plastic wrap, and let marinate for at least 30 minutes (or up to overnight for more flavor).

Sear the Beef: Heat the cooking oil in a hot wok or frying pan. Once sizzling, add the marinated beef in a single layer. Sear without stirring for about 30 seconds on each side until browned (3-4 minutes total).

Add Vegetables: Toss in halved cherry tomatoes or julienned bell peppers and sear for an additional 20 seconds. Remove from heat.

Plate the beef and vegetables over salad greens, garnished with pickled onions for that extra zing.