To start, combine the soy sauce, oyster sauce, fish sauce, sugar, black pepper, and 1 tablespoon of vegetable oil in a large bowl. Add the beef cubes to the bowl and mix thoroughly, ensuring each piece is well coated. Cover and marinate for at least 15 minutes, or up to 1 hour for more flavour.

While the beef marinates, prepare the salad by arranging the watercress, cucumber slices, and tomato on a serving platter. This fresh, crunchy base complements the richness of the beef.

In a small bowl, mix together the salt, black pepper, and lime juice for the dipping sauce. Adjust the lime to taste; this tangy, peppery sauce adds a refreshing contrast to the savoury beef.

Heat 1 tablespoon of oil in a large skillet or wok over high heat. Add the onion wedges and bell pepper, stirring for 2-3 minutes until they begin to soften. Remove and set aside. This brief stir frying retains the vegetables' crunch and bright colour.

In the same skillet, add the remaining tablespoon of oil and let it heat until it shimmers. Add the minced garlic and sauté for a few seconds until fragrant, being careful not to let it burn.

Add the marinated beef to the skillet in a single layer. Let it sear undisturbed for 1-2 minutes to achieve a nice brown crust. Then, shake or stir the beef to sear all sides evenly, cooking for about 3-4 minutes total for medium-rare. Avoid overcooking to keep the beef tender.

Return the cooked onion and bell pepper to the skillet with the beef, tossing everything together briefly. The vegetables should just heat through and absorb some of the beef juices, enhancing the dish's flavour.

Arrange the shaking beef and vegetables over the watercress salad on the serving platter. Serve with the lime-pepper dipping sauce on the side. For a traditional touch, enjoy with steamed jasmine rice or crusty French bread to soak up the savoury juices.