Roughly mash the boiled potatoes. Beat in the butter and chopped onion and place some mash into a round, metal ring. Dust the potato with flour and pan-fry for a few minutes on each side until golden brown.
Cut the monkfish tails in half and season with salt and pepper. Heat a little oil in an ovenproof pan and sear the fish in it for 2-3 minutes on each side and transfer the pan to the preheated oven and roast for another 8 minutes.
Roast the peppers over an open flame until the skins have burnt. Peel off the blackened skin and slice the peppers lengthways. Put the peppers in a saucepan with the vinegar, sugar and cashew nuts and cook for a few minutes.
To serve, place the pandy on the plate and arrange the fish on top. Spoon some of the pepper and cashew nut sauce on the plate.