Place the steak on a large plate and sprinkle with salt and pepper on all sides.
In a nonstick skillet, over high heat, heat 1 tablespoon of the oil. Add the steak and cook 4 to 5 minutes on each side for medium rare. Remove from 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 vinegar and cook 2 minutes more.
In a large bowl, mix the watercress with the orange sections and the remaining 2 tablespoons oil.
Divide the steak evenly among 4 shallow bowls and top with the glazed onions. Serve with the watercress and orange sections.