Place the steak on a large plate and sprinkle with the salt and pepper on all sides. In a large nonstick skillet, over high heat, heat 1 tablespoon of the olive oil. Add the steak pieces and cook 4 to 5 minutes on each side for medium rare. Remove from the pan and cover to keep warm. Reduce heat to medium-high and add the onion, tossing occasionally until crisp-tender and lightly browned, about 5 minutes. Add the balsamic vinegar and cook 2 minutes more. In a large bowl, mix the watercress with the orange sections and the remaining olive oil. Divide the steak evenly among 4 shallow bowls and top with the glazed onions. Serve with the watercress and orange sections.