Pan fry the guanciale: In a fry pan over medium heat, cook the guanciale until golden and crisp. Remove and set aside, leaving the fat in the pan.
Prepare and sear the meat: Lightly coat each piece of osso bucco with flour, salt, and pepper. In the same pan with guanciale fat, sear osso bucco for 2–3 mins per side until browned.
Build the sauce: To the slow cooker, add carrots, celery, onion, garlic, tomato paste, passata, rosemary, thyme, lemon rind, osso bucco and guanciale. Pour in red wine and chicken stock until the meat is almost submerged.
Slow cook: Cover and cook in the slow cooker on low for 8 hours, until meat is fall-apart tender.
Serve with your favourite sides (i.e., creamy mash potato and fresh gremolata).