Pat the beef cubes very dry with paper towels. Toss with the oyster sauce, soy sauce, fish sauce, sugar, half the garlic, and 1 tsp of the black pepper. Marinate at room temperature for 20 to 30 minutes while you prep everything else.

Make the dipping salt: stir the salt and remaining 1/2 tsp black pepper together in a small saucer, then squeeze in the juice of half the lime. Cut the rest of the lime into wedges.

Arrange the watercress, cherry tomatoes, and most of the red onion on a serving platter. Keep a little onion back for the pan.

Heat a wok or heavy skillet over the highest heat until it just starts to smoke. Add the oil, then half the beef in a single layer. Leave it alone for 60 to 90 seconds to build a crust, then shake the pan and sear another 1 to 2 minutes until browned on most sides but still pink inside. Transfer to a plate and repeat with the second batch.

Return all the beef to the pan with the reserved onion. Add the butter and remaining garlic and toss for 30 seconds until the beef is glossy and the garlic smells toasted, not burnt.

Pile the beef and pan juices over the salad. Rest 2 minutes so the juices dress the greens, then serve with the lime pepper salt for dipping.