To make the prosciutto-wrapped hoki, heat the oven to 200C/400F/Gas 6. Wrap the prosciutto around the hoki.
Heat the oil and pan fry the prosciutto-wrapped fish for two minutes on each side. Place in the oven and roast for five minutes or until cooked through.
To make the sweet potato mash, boil the sweet potato until soft, drain and mash with all the other mash ingredients.
To make the herby mayo, put the herbs in a mini blender and blend until smooth. Add the egg yolk, mustard powder and vinegar and gradually add the vegetable oil until it emulsifies and turns thick. Season with salt and freshly ground black pepper.
To serve, spoon a pile of sweet potato mash onto a large plate, top with the hoki and serve the herby mayo on the side.