Sprinkle the oregano over the steaks, along with a little seasoning. Vacuum seal and cook at 60C for 48 hours.
An hour before the osso buco is ready, get 125ml water in a saucepan and get boiling. Whisk in the polenta, pouring in a smooth stream. Add a pinch of salt and cook for another minute until thick. Stir in the cheese and tomato, then pour into an oiled baking dish. Put aside for a moment.
Over a gentle heat in a little butter fry the mirepoix. Add the stock and bouquet garni and simmer for 30 minutes. Remove the herbs and blitz with a hand blender, and keep warm whilst everything else finishes.
Get the grill on high and a pan on a high heat. Grill the polenta for around 10 minutes until crisp on top. Remove the osso buco and pour the cooking juices into the sauce. Pan fry the osso buco quickly on all sides until browned. Serve with slices of crisp polenta and some green beans.