1 1/4 pound tri-tip (bottom sirloin/culotte) steaks
1/2 teaspoon freshly cracked black pepper
1 1/2 teaspoons sugar
2 cloves garlic, minced
2 tablespoons oyster sauce
1 teaspoon fish sauce
1 tablespoon light (regular) soy sauce, or 2 teaspoons light (regular) and 1 teaspoon dark (thick) soy sauce
1 shallot, thinly sliced (1/4 cup total)
1 1/2 teaspoon sugar
1 or 2 pinches salt
3 to 5 cracks black pepper
1 1/2 tablespoons unseasoned rice vinegar
2 tablespoons water
4 cups watercress, use only the tender leafy parts
2 tablespoon canola or peanut oil
Trim excess fat from the steaks and then cut each into 3/4-inch cubes. In a bowl, combine the pepper, sugar, garlic, oyster sauce, fish sauce and soy sauce. Add the beef and toss well to coat. Set aside to marinate for 20 minutes or up to 2 hours.
For the dressing, put the shallot in a mesh strainer and rinse under water for about 10 seconds to reduce some of the harshness. In large mixing bowl, combine the sugar, salt, pepper, vinegar and water. Stir to dissolve the sugar. Add the shallot. Put the watercress on top but hold off on tossing.
Heat the oil in a wok or large skillet over high heat. Add the beef and spread it out in one layer. Cook in batches, if necessary. Let the beef sear for about 1 minute, before shaking the wok or skillet to sear another side. Cook for another 30 seconds or so and shake. Cook the beef this way for about 4 minutes total, until nicely browned and medium rare.
In between shakes, toss the watercress and transfer onto a platter or serving dish. When the beef is done, pile the beef on top of the watercress and serve immediately with lots of rice.
Use both the light and dark soy sauces if you want a little extra deep color. Feel free to dress up the final platter with some tomato wedges. If serving without the watercress, opt to present the beef with a side of salt, pepper, lime dipping sauce (muoi tieu chanh) for guests to dip the cubes in.