1. Preheat the grill to high. Mix all the dressing ingredients together. Spread generously onto the slices of sourdough bread.
2. Place the salt beef on top, 3 slices per slice of bread followed by the sliced gherkins and the Gruyère cheese.
3. Heat the butter in a large ovenproof frying pan until melted. Place the 2 sourdough slices into the pan and cook the underside of the bread for 2 minutes, until golden and crispy.
4. Transfer the pan to the grill and cook for 2-3 minutes until the cheese is melted and bubbling. Place each slice onto a serving plate, top with the watercress and sprinkle over some ground black pepper to serve.