Preheat the oven to 170C/340F/Gas 3.
For the sage butter, chop the sage leaves finely.
Place the butter in a bowl, add the chopped sage leaves and mustard, and season, to taste, with salt and freshly ground black pepper. Mix together well.
For the chicken, using a sharp knife, slice into the side of each chicken breast to form a pocket.
Spoon a quarter of the butter mixture into each pocket and fold to enclose. If you have any of the flavoured butter left over you can roll it into a sausage shape, freeze and use (within a month) in another dish.
Place two prosciutto slices on a clean board so that they overlap slightly. Wrap one of the chicken breasts in the slices and roll to enclose. Repeat with the remaining prosciutto slices and chicken breasts.
Heat the olive oil in a heavy-bottomed frying pan. When hot, add the wrapped chicken breasts.
Cook over a medium heat until the prosciutto is lightly golden, about two minutes.
Turn the breasts over and brown the other side for another two minutes.
Using tongs or a fish slice, place the chicken breasts onto a baking tray and place into the preheated oven. Retain the cooking juices in the pan, as these will be used to make the sauce.
Cook the chicken breasts for 20 minutes, or until the chicken is completely cooked through (the chicken will feel firm to the touch and the juices will run clear when the chicken is pierced with a skewer in the thickest part).
When the chicken is cooked, remove from the oven and put the breasts to one side to rest for ten minutes. Keep warm.
While the chicken is resting, make the sauce. Heat the retained cooking juices in the pan the chicken breasts were cooked in and add the white wine. Crush the garlic clove and add to the pan. Add a sage leaf.
Turn up the heat and cook, scraping up the brown bits from the bottom of the pan. Cook for a couple minutes until the sauce is reduced. Season, to taste, with salt and freshly ground black pepper.
To serve, slice each breast into five slices on the diagonal, then pour the mustardy, buttery sauce over. Serve with mashed potato.