After pounding out the veal, dust (don't dredge) with pinches of flour and season with salt and pepper. Place a slice of prosciutto along one side of each of the veal slices, and top with a sage leaf. Fold the veal over to cover the prosciutto and sage (not necessarily completely) and pound along the edge to seal. (You can use a toothpick to hold this pocket together if necessary.)
Heat the olive oil until it shimmers and is fragrant in a large heavy skillet over medium-high heat. Cook the veal about 20 seconds on each side and remove, doing four at a time.
Add the Marsala to the pan and bring to a boil, while dissolving off the browned bits from the bottom of the pan. Reduce the liquid to about one-half the original amount, then remove the pan from the heat. Whisk in the butter in small pieces to give a creamy look to the sauce. Adjust the seasoning with salt and pepper.
Return the veal to the pan just long enough to coat with the sauce, and serve immediately. This is great with a bitter green vegetable (broccoli di rape, or just plain broccoli) or wild rice.