In a large bowl, combine the garlic, sherry, soy sauce, sugar, pepper and five-spice powder with the 1 tablespoon of oil. Add the beef and turn to coat thoroughly with the marinade. Cover and refrigerate for at least 5 hours or overnight.
In a small bowl, combine the soy sauce with the vegetable oil, sherry, red wine vinegar, minced ginger, sugar, garlic and lemongrass. Let stand for 30 minutes, stirring a few times.
Mound the watercress and red onion on a platter and toss with half of the vinaigrette. Set 2 large skillets over high heat until very hot, about 2 minutes. Add 1 tablespoon of oil to each skillet; when shimmering, add the meat to both skillets in an even layer. Let cook undisturbed, until richly browned on the bottom, about 1 1/2 minutes. Shake the skillet to release the meat and cook for about 30 seconds longer, until medium-rare. Scrape the beef onto the watercress. Drizzle with the remaining vinaigrette, toss lightly and serve right away.