In a pestle and mortar or Flavour Shaker, smash up 2 teaspoons of salt with the lemon zest and rosemary and rub this all over the fish fillets. Put the fillets in a dish in the fridge and let them sit there for an hour.
Now make your black olive sauce by mixing all the ingredients except the vinegar together. You want the sauce to have the consistency of a coarse salsa. Then carefully balance the flavours with the vinegar to taste.
If you’re roasting your monkfish, preheat your oven to 220°C/425°F/gas 7 just before the fish comes out of the fridge. Pat the fish dry with some kitchen paper and then pat it with a little olive oil.
Peel and halve your potatoes. Put them into a pot of salted, boiling water and cook until tender. Then drain and mash up with 6 tablespoons of olive oil and a good swig of milk. Season to taste with salt, pepper and lemon juice. If you want to get your mash really smooth and creamy you can use a spatula to push the potato through a sieve once or twice. It doesn’t make it taste any better but it will make it silky smooth, shiny and lovely. Just depends if you can be bothered, really. If it needs thinning with a little extra milk, feel free.
To roast the monkfish, heat a large ovenproof frying pan, add a splash of olive oil and fry the fillets in the pan for 2 minutes. Then turn them over and put the pan in your preheated oven for 6 to 8 minutes, depending on the thickness of the fillets.
To grill, place the the butterflied fillets on a hot griddle pan and cook for about 3 minutes on each side, depending on the thickness. Whichever way you cook it serve the fish and the juices with a good dollop of the mashed potato, the black olive sauce and a little rocket dressed with the extra virgin olive oil, lemon juice and salt and pepper. Really, really good.