Cover a chopping board with a piece of baking paper. Put the chicken breasts on the board, slice through each one horizontally and open out (as if you were reading a book). Put another piece of baking paper over the opened-out chicken breasts and, using a meat mallet, rolling pin or the base of a small heavy saucepan, pound the chicken a little to get even 1cm slices. Secure two sage leaves to each breast with toothpicks.
Put the flour on a plate and season with salt and freshly ground black pepper. Heat the oil in a frying pan until hot. Dredge the chicken in the seasoned flour and fry in the hot oil for about 3 minutes on each side. Set aside on a warm plate in a low oven to keep warm.
Add the marsala to the same pan and bring to the boil, scraping the pan. Whisk in the butter and any juices from the resting chicken and season. Return the chicken back to the pan along with the chopped sage and turn in the sauce. Serve with polenta or mashed potatoes and a green salad.