1. Preheat the grill pan or Panini press on medium.
2. In a bowl, mix together the red wine vinegar, extra virgin olive oil, salt and pepper.
3. Once well mixed, toss the watercress into the dressing.
4. Assemble the sandwiches by placing watercress, 3 slices of each type of cheese and 2 slices of tomato, inside. Spread butter on the outside of the sandwich and place on the grill pan. If not using a panini press, then put some weight on top of the sandwich to press. Grill both the side until golden brown.
5. Serve warm.
Have cheese on both ends of the bread so the sandwich sticks together.