Combine beef with 4 cloves garlic, fish sauce, sugar, salt and 2 teaspoons of olive oil. Mix and allow to marinate for 30 minutes. Marinate onion in vinegar for 10 minutes, then sprinkle on black pepper and 1 tablespoon of olive oil. Combine watercress with the onion and arrange on a platter. Heat the remaining teaspoon of olive oil in a skillet. Add remaining garlic and stir, then add the meat. Fry quickly until seared on the outside. Pour over the watercress and serve.