If using lardo or pork fat, mince it finely together with garlic on a cutting board. Sauté the resulting mince very gently in a large saucepan until the lardo/pork fat has rendered and the garlic has very lightly browned. If using lard, melt it in the saucepan and add the garlic, sautéing very gently until it had very lightly browned. Then add the pancetta and let it sauté gently as well for a few minutes.
Add the cabbage and turn it to mix well with the fat and pancetta. Let the cabbage sweat for a good 5 minutes or so, or until well wilted. It should have reduced in volume by about half.
Add enough broth to barely cover the cabbage. Cover and let everything simmer gently until the cabbage is fully tender, about 20-30 minutes.
In a gratin or baking dish with deep sides, lay out bread slices to cover the bottom. Ladle over some of the cabbage and its broth to cover the slices entirely. Sprinkle generously with grated parmigiano-reggiano and, if using, some of the fontina and a scrape of nutmeg. Repeat until all the ingredients have been used up, ending with with a final generous layer of cheese. NB: Add more broth or water if things look a bit dry; the liquid should still be visible and come up about 2/3 of the way up the casserole. And don't fill the baking dish to the brim since the zuppa will puff up in the oven a bit like a souffle. Bake in a hot (200C/400F) oven until the soup is bubbly hot and the cheese topping has melted to form a golden crust. The bread should have absorbed most but not all of the broth during cooking.
Serve piping hot.