Pat tofu dry. Heat 1 tablespoon vegetable oil in a 12-inch nonstick skillet over moderately high heat until hot but not smoking, then saute tofu until golden brown, about 3 minutes on each side. Transfer to a plate. Heat remaining 1/2 tablespoon vegetable oil in skillet over moderate heat until hot but not smoking, then cook watercress, turning with tongs, until just wilted. Stir in sesame seeds. Transfer watercress to a platter and arrange tofu on top. Simmer remaining ingredients in skillet 1 minute and drizzle sauce over tofu.