How to make
To make the bag you're going to need a large sheet of baking paper and a stapler. Fold the sheet in 2, fold in the opposing open sides and staple them in several places.
Clean the fish and cut them in 2 horizontally, salt them to taste, sprinkle black pepper, rosemary and freshly squeezed lemon juice. Leave them aside while you prepare the vegetables.
Peel and clean the potatoes, onions, celery, eggplant and carrots, cut all of them into strips using a potato chopper.
Put them in a bowl, add the finely grated garlic cloves, chopped parsley, washed and chopped into strips peppers and the oil. Salt to taste and stir well.
Put the paper bag you made into a tray, smear its insides - top and bottom - with the soft butter. Place it lying flat and first arrange the mixed veggies, then the 4 pieces of mackerel on top of them.
Close the bag - fold the paper along the opening inward, staple it.
Pierce the bag in several spots on top with a needle and place the tray in a preheated 390°F (200 °C) oven to bake for 40 min.
