Preheat oven to 160C. Heat olive oil in a heatproof casserole dish, add onion and garlic and sauté over medium heat for 5 minutes or until translucent, add carrot and celery and cook, stirring, for 3 minutes or until softened slightly. Add veal and pork, then add wine, rosemary and 250ml tomato passata. Cover and bake for 2½ hours or until meat is tender. Cool, remove meat and shred using two forks, then return meat to sauce and stir through.
For besciamella, melt butter in a saucepan, sift in flour and stir over medium heat until smooth and starting to colour. Warm milk in a separate saucepan, then slowly whisk into flour mixture until combined. Stir over low heat for 5 minutes or until smooth and thick. Stir through parmesan and season to taste.
Preheat oven to 180C. Cut pasta into sixteen 12cm squares and cook in boiling salted water for 1 minute. Drain and spoon 1/3 cup meat sauce along one edge of each piece of pasta and roll into cylinders. Spoon remaining passata into the base of a 25cm x 35cm roasting pan and top with cannelloni. Spoon over besciamella and top with mozzarella. Bake for 35-40 minutes or until golden. Serve with a green salad.