1. In a bowl, add the cut beef.
2. Add the egg white and cornstarch and mix the meat in it well.
3. In a wok, heat oil.
4. Add the beef to the wok and saute for 2 minutes.
5. Drain the oil out.
6. Add some more oil to the wok along with garlic and onion.
7. Add the beef back into the wok.
8. Add oyster sauce, soy sauce, sugar, black pepper and scallions and saute.
9. Lay out the watercress on a plate.
10. Add the sauteed beef dish onto the watercress.
11. Garnish with cherry tomatoes, lime juice, salt and white pepper.