Preheat the oven to 200ºC and grease a large dish with oil. In my case I have used the same container to first cook the potatoes on the stove, but if you do not have a source that allows it, you will simply have to use any pan before moving the potatoes.
The best is ask the fishmonger to prepare our tail to fill it, but we can do it at home by carefully opening it in half and removing the central spine. If we use fresh prawns or prawns we can prepare a quick stock with the shells and heads, cooking them with the fish bone in water for about 15 minutes.
Wash the potatoes and turnips, peel them and cut into slices less than a finger thick. Fry lightly without much oil, letting them brown, turning them on both sides. Reserve the oil and place in the dish, seasoning them with salt and pepper. Bake for about 15 minutes with a splash of white wine or broth.
Chop the spring onion and wash and chop the mushrooms. Sauté the chives in the oil of the potatoes with a pinch of salt, add the mushrooms and cook over high heat until they reduce their volume by half. Add the chopped prawns, season and add the crumbled cayenne. Cook over medium heat, sautéing the whole, until the liquid is greatly reduced.
Open the hake, season with garlic and pepper and spread over the mushroom and prawn filling, reserving the pan juices. Do not go too far, if there is excess we can save it for another recipe. Close by pressing gently, and tie if we want with kitchen thread. Take to the source, water with a splash of white wine and a little broth (if we have it) and bake for about 20-25 minutes.
Reheat the pan where we have cooked the filling and add the juices from the source. Moisten with wine and a little broth or water. Mix with a heaping teaspoon of cornstarch diluted in cold water and cook over high heat stirring until thickened. Add fresh parsley and a dash of pepper. Sauté the hake divided into portions.