Melt the butter in a saucepan over a medium heat. Add the monkfish and cook for three minutes on each side.
For the crushed potatoes, crush the boiled and drained potatoes roughly with a fork and add the butter, Dijon mustard, parsley and chives and mix well.
For the shallot dressing, heat the chopped shallots and olive oil in a small pan. Warm through and remove from the heat.
Add the lemon juice and, once cooled, add the chives.
To serve, place the crushed potatoes in the centre of the plate. Place the monkfish on top and drizzle the dressing around.