Rinse the spinach and watercress under cold running water and drain thoroughly on paper towels.  Remove any tough stems.

Place the spinach and watercress leaves in a large serving dish.

Cut the mozzarella into small pieces and sprinkle them over the spinach and watercress leaves.

Cut the tomatoes in half and sprinkle them over the salad.

Sprinkle over the vinegar, oil and season to taste with salt and pepper.  Toss the mixture together to coat the leaves.

Serve immediately or let chill in the refrigerator until required.