||Make the dressing: Combine the onion, garlic, sugar, salt, vinegar, olive oil and black pepper to taste in a large salad bowl; mix well. Set aside.
||Prepare the beef: Cut the beef against the grain into thin 1 by 2-inch strips. In a bowl, combine the beef, half of the minced garlic (reserve remaining half for frying), fish sauce, soy sauce, sugar and ground black pepper to taste. Let stand for 30 minutes.
||Clean the lettuces and watercress. Drain and pat dry with paper towels. Add the greens to the dressing, but do not toss.
||Preheat a large skillet over high heat and add the vegetable oil. Fry the remaining minced garlic until fragrant and golden. Add the beef and saute quickly, shaking the pan, over high heat to sear, about 1 minute (the beef should be medium-rare). Pour the seared meat over the greens and toss gently. Divide the warm salad among 4 individual plates.
||Sprinkle with freshly ground black pepper and serve with French bread, if desired