Preheat the oven to 180°C conventional (160°C fan-forced).
Toss the veal or beef in the seasoned flour and shake away any excess.
Heat 2 tablespoons of olive oil and the butter in an ovenproof cast iron pan over medium–high heat. Cook the veal or beef in batches for about 1–2 minutes or until browned on each side. Remove and set aside.
Heat 1 tablespoon of olive oil in the same pan, add the onion and cook for 1–2 minutes. Add the garlic, carrot and celery and cook, stirring, for 5–6 minutes.
Return the veal or beef to the pan, add the wine, passata and stock and season well. Bring to the boil, cover with a lid and bake for 3 hours or until the veal or beef is very tender.
For the gremolata: Place all the ingredients in a small bowl and combine well.
Serve the osso bucco on mash, polenta, rice or couscous and top with the gremolata.