Preheat the oven to 200C/400F/Gas 6.
Season the monkfish with some salt and set it aside for 15 minutes.
Cook the potatoes in well-salted boiling water until tender. While the potatoes are cooking, heat the 2 tbsp/1fl oz/30ml of olive oil in a large, ovenproof frying pan.
Pat the monkfish dry on kitchen paper, add to the pan and sear for 3-4 minutes, turning it 3 or 4 times, until nicely browned on all sides.
Transfer the pan to the oven and roast for 10-12 minutes, until the fish is cooked through but still moist and juicy in the centre. Remove from the oven, cover with foil and set aside for 5 minutes.
When the potatoes are done, drain them well and return them to the pan with the extra virgin olive oil. Gently crush each potato against the side of the pan with the back of a fork until it just bursts open. Season with salt and pepper, add any juices from the fish and the watercress and turn over gently until the watercress is well mixed in.
Cut the monkfish across into thick slices.
Spoon the crushed potatoes on to 4 warmed plates and put the monkfish on top.
Put your thumb over the top of the bottle of extra virgin olive oil and drizzle a little of it around the outside edge of each plate.
Do the same with the balsamic vinegar and then sprinkle around a few sea salt flakes and coarsely crushed black pepper.
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