First make your 4 tinfoil or greaseproof paper bags. Do this by taking 4 x A5-sized pieces of foil or greaseproof paper, fold each one in half then fold up the 2 ends tightly, giving you an ‘envelope’. Preheat the oven to 250ºC/475ºF/gas 9. Boil your spuds until cooked, then drain and put to one side to cool. Slice them up roughly. Peel back the artichoke leaves, remove the chokes with a teaspoon, then rub the artichokes with lemon juice to stop them discolouring and slice very finely. Put the artichokes and potatoes in a bowl with the melted butter, thyme, parsley and capers and season well. Toss together and divide between the bags. There'll be a little butter left over in the bottom of the bowl, so you can add a little olive oil to it and use this to rub on to the skate pieces.
Season the fish then place on top of the potatoes and artichokes. Add a glass of wine to each bag with a sprig of rosemary, tightly sealing the final side and pulling up the corners so that the liquid is contained. Bake on a roasting tray in the preheated oven for 15 minutes. By this time the bags will be beautifully puffed up. Serve at the table so that your guests can open their bags themselves, leaving a plate of really flavoursome vegetables, fish and juice. Serve with a dollop of crème fraîche, seasoned with a little salt and pepper, and a nice green salad.