For the monkfish, crush the pistachio nuts in a pestle and mortar. Place the crushed pistachios into a bowl with the flour, sesame seeds, salt and freshly ground black pepper and mix well.
Spread the mixture out on a baking sheet and roll the monkfish in the mixture to coat all over.
Heat the butter and oil in a frying pan and fry the fish until golden-brown and crisp on all sides, about 6-7 minutes.
Meanwhile, for the crushed potatoes, bring a pan of water to the boil and cook the potatoes until tender, about 10-12 minutes. Drain and crush using the back of a fork. Stir in the butter and the chives.
For the sauce vierge, heat all the ingredients in a pan until the tomatoes begin to break down.
To serve, cut the monkfish in half. Place the potatoes onto a serving plate, top with the monkfish and drizzle over the sauce.