Step 1Cut the beef into 1-inch cubes.
Step 2Prepare the marinade in a bowl or container with a lid by combining 2 tablespoons of the oil, the fish sauce, black pepper, oyster sauce, garlic and sugar. Mix well until the sugar is dissolved, then add the beef cubes. Cover the bowl or container and shake the cubes to evenly coat the meat (or you can simply stir). Leave the cover on and let the container sit for 20 minutes on the counter.
Step 3Clean the watercress and arrange it on a large serving platter or dish.
Step 4In a large skillet, heat the remaining 3 tablespoons oil over high heat. When it is hot, add the onion. Saute for just a few minutes, then throw in the beef with its marinade and toss quickly. You need to cook for only 5 minutes over low to medium heat for the meat to be medium rare; continue tossing as it cooks. Cook it longer if you prefer.
Step 5When the meat is cooked, turn off the burner and stir in the cornstarch to thicken the sauce. Spoon onto the watercress and top with tomato wedges. Serve family style with steamed rice.