Trim and pat dry the Beef tenderloin, then cut into roughly 1-inch cubes so they sear evenly.
In a mixing bowl combine Oyster sauce, Soy sauce, Fish sauce, a little Sugar, a drizzle of Canola oil and Cornstarch; add a generous pinch of freshly ground Black pepper and toss the beef to coat. Let the meat marinate briefly while you prepare the salad and dressing (15–30 minutes).
Make the salad base by thinly slicing the Red onion, halving or quartering the Tomatoes, slicing the Cucumber and placing them with the Watercress in a large bowl.
Whisk together the dressing: Rice vinegar with a touch of sugar, a pinch of Fine salt, a little more Canola oil and the juice of a Lime. Adjust to taste so it’s bright and slightly sweet-tangy.
Heat a heavy skillet or cast-iron pan over high heat until very hot. Add a splash of Canola oil and a knob of Unsalted butter to the pan so the butter browns but doesn’t burn.
Working in batches so the pan isn’t crowded, sear the marinated beef cubes in a single layer until deeply browned on all sides. Near the end of each batch, add minced Garlic to the pan and toss quickly so the garlic becomes fragrant without burning. Remove the cooked beef to a plate and repeat with remaining meat.
Once all the beef is cooked, briefly return all pieces to the hot pan to glaze with any pan juices and a little extra butter if desired, then remove from heat.
Toss the salad with the dressing, taste and adjust seasoning with more Fine salt or Black pepper if needed. Plate the dressed watercress and vegetables, top with the warm shaking beef, and finish with a squeeze of Lime over the whole dish.
Serve immediately so the beef stays juicy and the salad stays crisp — the contrast of warm, caramelized meat and cool, tangy greens is the hallmark of a great Bò Lúc Lắc.
