Prepare the mash using the anchovies, lemon juice, garlic, parsley, capers, salt, pepper and 4 tablespoons of olive oil. Marinate the fish in this for 15 minutes.
Cut a piece of aluminum foil that is large enough to hold the fillets of fish, then spread a little butter over one side of it.
Place the fillets of fish in the middle of the buttered foil with the remaining mash spread over the top. Fold over the foil and seal together the edges of the foil.
Place the foil on a baking tray and put in the oven and cook for around 25/30 minutes at 200 C.
Open foil at the table to allow your guests to enjoy the aroma of this dish
Serve with side dishes of steamed mixed vegetables and potatoes drizzled with olive oil and salt.