Preheat the oven to 170°C.
Finely chop the sage leaves for the butter.
Mix softened butter with chopped sage, mustard, salt, and pepper.
Cut a pocket into the side of each chicken breast portion.
Fill each pocket with the flavored butter and fold to close.
Wrap each stuffed breast tightly in two slices of prosciutto.
Heat olive oil in a heavy-bottomed frying pan.
Sear the chicken over medium heat for two minutes until golden.
Brown the other side of the breasts for an additional two minutes.
Transfer chicken to a baking tray, keeping the pan juices for later.
Oven-roast the chicken for 20 minutes until juices run clear.
Remove from oven and rest in a warm place for ten minutes.
Add wine, crushed garlic, and a sage leaf to the pan juices.
Reduce the sauce over high heat while scraping the pan bottom.
Slice chicken diagonally and serve topped with the pan sauce.