Preheat oven to 220C. For cotechino stuffing, heat oil in a frying pan, add onion and sauté over low heat for 12 minutes or until soft. Add celery and cook over medium heat for 3 minutes. Remove casing from sausage and crumble meat into frying pan, add sage and cook for 5 minutes. Add red wine and simmer for 5 minutes or until reduced by half. Season to taste with sea salt and freshly ground black pepper. Cool, then spoon evenly into the cavities of squab.
Season squab with sea salt and freshly ground black pepper and place in a roasting pan. Drizzle with olive oil and roast for 20 minutes or until golden brown. Remove, cover with foil and rest for 10 minutes.
Meanwhile, heat butter in a large saucepan until foaming, add onion and silverbeet stems and sauté for 10 minutes or until soft, add cream and simmer for 10 minutes. Add silverbeet leaves and cook for 5 minutes or until wilted, season to taste. Serve squab with silverbeet, drizzled with pan juices.