How to Make It
Stir together oyster sauce, soy sauce, sugar, cornstarch, fish sauce, pepper, and garlic in a medium bowl. If a saltier finish is desired, add up to 1 1/2 teaspoons more oyster sauce. Add beef, toss well to coat, and let marinate 20 minutes at room temperature.
Rinse onions in a strainer under cold running water for about 10 seconds; set aside. Whisk together 2 tablespoons water, vinegar, sugar, salt, and pepper in a large bowl until sugar is dissolved. Add onion; top with watercress, radicchio, and herbs. Do not toss.
Heat a large, heavy skillet over high, and add oil. When oil is shimmering, carefully add beef in a single layer. Cook, shaking pan every 30 to 60 seconds, until seared on all sides and meat reaches desired degree of doneness, 3 to 4 minutes for medium-rare. Remove from heat. Quickly toss salad, and transfer to a platter or serving dish. Pile cooked beef and juices on salad, and serve immediately.