Pound the cutlets well with a meat pounder or the flat of a wide knife frequently dipped in water to thin them.
Mix the remaining veal, the prosciutto, the marrow, and the cheese together, adding the egg last to bind the mixture; season it with a pinch of pepper (salt shouldn't be necessary because of the prosciutto and the Parmigiano). Stretch the cutlets out and spread the mixture over them, then roll them up and tie them with string.
Mince the onion, celery, carrot, and pancetta. Melt the butter in a skillet over a medium flame and, when the onion's lightly browned, add the veal cutlets, seasoning them with salt and pepper.
When the cutlets are browned, add the tomato sauce and a little water (or the wine), cover partially, and simmer them till they're done, (15-20 minutes, though I usually go by eye).
Remove the strings before serving.
These stuffed cutlets can also be roasted in the oven. While you're preparing them, preheat the oven to 350 F. Put the cutlets on a rack, baste them with olive oil and salt, and roast them till they're done (about a half hour), turning and basting them once or twice.
The recipe will serve four to six as a second course or two to four as a main course.