Saute onion in half of the butter in a wide shallow skillet until soft and golden. Dust the veal shanks with flour, and then fry them in the same skillet, turning several times until they are golden on all sides. Stand them on their side to prevent the marrow in the bones from slipping out during cooking.
Add the fresh mushrooms, carrots, celery and season with salt and pepper. Add sage and simmer for 4 minutes. Add the wine and let evaporate. Add the tomatoes and veal stock and simmer gently for about 1 hour, or until cooked and tender. Add more wine if sauce reduces too quickly.