Put a large pan of salted water on to boil. Peel and chop the potatoes into small chunks. When the water is boiling, add the potatoes and simmer for 15-20 mins till soft.
While the potatoes simmer, trim the ends off the leeks and slice a cross in the tops. Rinse out any grit. Finely slice them. Pick any thick stalks out of the watercress. Roughly chop the watercress. Heat your grill to high.
Warm a pan over a medium heat. Add 1 tbsp butter and the leeks with a pinch of salt and pepper. Cook and stir for 10 mins till the leeks have softened. Stir 1 tbsp cornflour into the pan. Cook and stir for 1 min.
Add 1 tbsp mustard to the pan. Pour in the cream. Add the watercress. Bring to the boil, stirring to wilt the watercress, then take off the heat.
Roughly chop the cod into bite-size chunks. Pop it into an ovenproof dish. Pour over the creamy watercress sauce. Grill for 10-12 mins till golden and bubbling.
While the cod grills, drain the potatoes well, then tip back into the pan and add the rest of the butter with a pinch of salt. Mash till smooth.
Scoop the mash into bowls and top with the grilled cod and watercress sauce.