To make the stuffing, combine the ground veal, pork and beef in a large mixing bowl, and add the bread crumbs, onions, water, eggs, 1 tablespoon of salt and 1 teaspoon of pepper. Mix with your hands or a large spoon until all the ingredients are well combined. Then vigorously knead the mixture for 3 to 5 minutes, or until smooth.
Preheat the oven to 350 deg. F. Skin-side down, spread the veal should flat on a table and, with a small, sharp knife, make small cuts in the thickest areas of the meat so that it lies even flatter. Lay a sheet of wax paper over the veal and with the side of a cleaver or meat pounder, pound the meat to a fairly uniform thickness. Remove the paper and sprinkle the veal liberally with salt and conservatively with pepper.
Spread half the stuffing on the veal, leaving a 2-inch border of the veal exposed all around the sides. Lay the hard-cooked eggs in a row down the length of the stuffing and spread the remaining stuffing in a layer over them. Bring one long side of the veal over the filling to the middle and tuck in the two ends. Now bring the other side over the filling, enclosing it snugly. With a strong kitchen cord, tie the rolled veal crosswise at 2-inch intervals, then with more cord tie it lengthwise.
Place the rolled veal seam-side down in a shallow roasting pan just large enough to hold it comfortably. Pour in 1 ½ cups of cold water or chicken stock and roast for 2 hours, undisturbed, basting the veal from time to time with the juices in the pan.
When the meat is a deep golden brown, carefully transfer it to a serving platter and cut off and discard the cords.
Bring the juices remaining in the pan to a boil over high heat. If most of the liquid has cooked away, add the remaining ½ cup of water or chicken broth to the pan, meanwhile scraping into it any brown bits that may be clinging to the bottom and sides of the pan. Off the heat, stir the ½ cup of sour cream into the sauce, a tablespoon at a time. Taste for seasoning and pour into a sauce dish.
Slice the roll crosswise into 1-inch rounds and arrange them slightly overlapping down the center of a large heated platter. Moisten them with a few tablespoons of sauce and serve the remaining sauce separately.