Preheat oven to 180°C or 160°C fan. Cut open chicken breasts butterfly-style and place on a chopping board. Cover each fillet with a sheet of plastic wrap. Using a rolling pin or meat mallet, gently beat down chicken until as thin as possible without breaking flesh.
Combine garlic, sage and parsley in a small bowl. Smear mustard on inside of each fillet. Top each with prosciutto and garlic mixture. Starting with a long edge, roll up each fillet as tightly as possible and tie neatly with string in several places.
Melt butter in a flameproof baking dish on medium heat. Add chicken and cook for 3-4 minutes, until golden all over. Pour over dry sherry and bake for 10 minutes, until juices run clear when pierced with a skewer.
Remove chicken from oven, cover with foil and set aside for 5 minutes. Remove string. Slice each roll into 2cm-thick slices and arrange on warmed dinner plates, spooning cooking juices over. Serve with mashed potato and green salad.
Tip: if you're cooking for six people instead of four, simply use 6 half chicken breast fillets and 3 slices of prosciutto, then add a little chicken stock to the sauce to stretch it further.