Step 1 Cut around the cabbage core and remove it. Under running water, gently remove each leaf from the head. You will need 16. Heat the chicken stock in a large saucepan. Simmer the leaves in the chicken stock, in batches, for 7 to 8 minutes each batch, until tender. Remove the leaves from the pan; set aside to cool.
Step 2 Cut away the tough rib on each leaf. Stack the leaves into pairs on a work space and set aside.
Step 3Heat 2 tablespoons of the oil in a large skillet over medium heat. Crumble the sausage into the pan and cook until browned. Transfer to a mixing bowl and let stand until cool.
Step 4Wipe out the skillet and add the remaining olive oil. Heat the oil over medium heat. Add the onion, sprinkle with one-fourth teaspoon salt and cook, stirring often, until the slices are soft, about 10 minutes. Add the tomatoes with their liquid and break up with a wooden spoon. Cook, stirring occasionally, for 15 minutes. Cool slightly, then spread into a 9-by-13-inch baking dish.
Step 5Heat the oven to 375 degrees. Add 1 cup of the Fontina and the bread crumbs to the sausage and mix well. Season with one-fourth teaspoon salt and one-eighth teaspoon pepper. Divide the mixture among the cabbage leaves, mounding about one-third cup in the center of each stack. Carefully roll up to enfold the filling by folding in the bottom first, then the two sides, then rolling it up like a burrito. Lay each packet seam side down on the sauce in the pan. Sprinkle evenly with the remaining cheese.
Step 6Bake 15 to 20 minutes. Serve each roll on a little of the sauce.