Cotechino or zampone sausage is the big hero in this dish. You should be able to buy these, or order them at least, from any good Italian deli (and if they can't get them for you then they're not a good Italian deli). They come already cooked in a vacuum-sealed bag. If you pop them out of the bag they can be roasted happily with the onions for quite a long time, and have a wonderful flavour, but if you can't get hold of one I'd suggest simply cutting open a good-quality fresh Italian-style or coarse sausage and breaking it into the basic risotto at the beginning when frying off your onions and celery.
Preheat the oven to 190ºC/375ºF/gas 5. Place the cotechino on a small roasting tray with the unpeeled onions and pop into the preheated oven. Bake for about an hour until the onions are soft and sweet and the sausage is cooked. Remove everything from the oven and turn it down to 150ºC/300ºF/gas 2.
Start the risotto bianco
as usual and continue through the recipe, adding the stock little by little. While doing this, you'll have plenty of time to peel the cooled onions. Cut one of them into quarters, put them in a dish and keep them warm in the oven. Finely chop the other 2 onions, peel the thick fatty skin off the sausage and discard it. Crumble up the sausage meat inside. When the rice is three-quarters cooked, at Stage 3, stir in the chopped baked onions and the crumbled sausage, and continue cooking the risotto until the rice is perfectly cooked.
At Stage 4, when you add the butter and Parmesan, taste, season carefully to perfection, and fold in the chopped thyme leaves. Divide between your plates, break up one of the onion quarters over each, and serve sprinkled with grated Parmesan.
PS This is also good with cooked borlotti beans stirred in at the end.