Whisk garlic, oyster sauce, 1 1/2 tablespoons sugar, fish sauce, sesame oil, soy sauce, and hoisin sauce together in a bowl; add beef. Marinate beef in refrigerator for at least 1 hour.
Whisk vinegar, 1 1/2 tablespoons sugar, and 1 1/2 teaspoons salt together in a bowl until vinaigrette is smooth; add onion. Refrigerate for 10 minutes to pickle the onion.
Whisk lime juice, 1/2 teaspoon salt, and black pepper together in a bowl until dipping sauce is smooth. Transfer dipping sauce to ramekins.
Heat cooking oil in a wok or large skillet over high heat until oil starts to smoke; add beef. Cook in a single layer until beef is seared, about 2 minutes. Cook and stir (or "shake") until beef reaches desired doneness, 2 to 4 minutes.
Spread watercress onto a serving plate and top with tomatoes. Drizzle vinaigrette over tomatoes and layer beef onto watercress; top with onion. Serve dipping sauce on the side.