1. In a nonstick skillet over medium-high heat, sear beef 5 minutes on each side for medium-rare, or until desired doneness. Season to taste with salt and pepper and let cool slightly.
2. Meanwhile, in a large bowl, whisk together vinegar, olive oil and mustard; season with salt and pepper. Add watercress and toss to coat. Evenly divide cress among four plates.
3. Thinly slice steak into 20 pieces; arrange atop cress. Sprinkle with mint, feta and tomatoes.