Grill the slices of bread on a griddle pan until lightly toasted on both sides then spread one side of each with mayonnaise. Put some of the sauerkraut and some of the chilli on 2 of the slices, and top with a couple of slices of pastrami. Top with the remaining sauerkraut and chilli and the sliced gherkins, then grate the Swiss cheese over the top.
Preheat a hot grill. Place the slices with toppings under the grill until the cheese is melted and dribbling.
Stack the sandwich together, adding a few watercress leaves and finishing with the final slice of toast. Press down lightly and use wooden skewers to hold together.