Cook the potatoes in well salted boiling water until tender.
For the court bouillon, put all the ingredients into a large shallow pan. Bring to the boil and simmer for 20 minutes.
Cut each skate wing in half. Add them to the court bouillon and simmer gently for 10 minutes.
Meanwhile, for the sauce, steep the saffron in a couple of teaspoons of warm water for five minutes. Then put the saffron and its water, tomatoes, coriander, cumin, red pepper, chilli, garlic, oil, lemon juice, one teaspoon of salt and ten turns of the black pepper mill into a small pan and, just before the skate is ready, warm the sauce through over a very gentle heat.
Lift the pieces of skate out of the court bouillon, drain off the excess liquid and set aside on a plate. Stir the coriander and mint into the sauce, spoon some over each piece of skate.
Heat the two tablespoons of olive oil in a large frying pan. 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 the potatoes and add any juice from the fish. Add the watercress and turn over gently until the watercress is well mixed in.
To serve spoon the crushed potatoes onto the four warmed plates and put the skate 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.