Preheat the oven or grill and cook the fish fingers following packet instructions.

Meanwhile, put the watercress into a food processor and whizz until very finely chopped. Add the mayonnaise and whizz again until the mayonnaise is green.

Spread a slice of bread thickly with the watercress mayonnaise. Top with the fishfingers, then the tomatoes and finish with a slice of bread. Serve hot with any remaining mayonnaise.