Step 1In a medium bowl, whisk together the garlic, ginger powder, sugar, vinegar, soy sauce, grapeseed oil and black pepper. Set aside. Makes slightly more than one-half cup.
Step 1In a nonreactive container, toss the beef with 1 1/2 teaspoons of the minced garlic, the fish sauce, soy sauce and sugar. Cover and refrigerate at least 2 hours, or up to overnight.
Step 2Heat a large saute pan or wok over high heat and add the oil. Add the beef and sear until browned on all sides, then remove and drain on paper towels.
Step 3Add the remaining 1 1/2 tablespoons minced garlic and diced onions and saute for 30 seconds, until aromatic.
Step 4Return the beef to the pan and add the oyster sauce, shaking the pan to distribute the flavors, then remove from the heat.
Step 5In a medium bowl, toss the watercress with the red onions and a scant tablespoon of the dressing (enough to just coat the greens). Arrange the salad on a plate and top with the cooked beef. Garnish with the chopped green onions; serve immediately.