Put the turkey in a bowl. Add all the remaining ingredients except for the mozzarella and work them in gently by hand. Divide the mixture into 12 equal pieces. Flatten each piece into a round patty. Top 6 of the patties with a piece of mozzarella and then with a second patty, making 6 polpettones. By hand, press the edges together to seal them and to make patties of even thickness. Sprinkle both sides of the patties with salt and pepper.
Heat a large ovenproof skillet over moderate heat. Add olive oil to a depth of 1/8-inch. When the oil is hot, add the 6 patties. Cook on 1 side until lightly browned, about 1 1/2 minutes, then turn and cook on the second side until browned, 1 to 2 minutes longer.
While the patties are cooking, heat the marinara sauce in a deep skillet. Add the cooked patties to the sauce. (Or drain the oil from the skillet and add the tomato sauce directly to the patties.)
Put a few tablespoons of grated mozzarella on top of each patty. Simmer the patties in the sauce for about 4 minutes, until the cheese is melted.
Assemble the sandwich: Put a few tablespoons of sauce and 1 polpettone on a split roll. Top with a little more sauce and serve immediately.
Michael's Notes: For the lightest polpettone, work the mixture as little as possible.