Cut meat into 3/4" cubes. Mixwith 2 cloves garlic, 1/2 tsp pepper, 1/2 tsp sugar and 1 tbsp oil. Let meat stand in marinade for 1 hour. Cut onion lengthwise into very thin strips. Marinate onion in vinegar, 1/4 tsp salt, 1 tsp sugar, 1/4 tsp pepper and 2 tbsp oil. Rinse watercress, drain well and arrange on a platter. Heat 2 tbsp oil over high heat. Add a pinch of sugar and the remaining garlic. Fry until fragrant. Add meat and stir fry for 2 minutes, until just cooked. Top watercress with meat, then onion mixture.