Mix the flour with salt and pepper on a chopping board. Cut the ox cheeks into very large pieces, at least 6–8 cm across, and roll them in the seasoned flour until they are well coated. Heat the oven to 150°C.
Heat the olive oil in a casserole and fry the pieces of ox cheek until they are well browned on all sides. Remove them to a dish, turn down the heat and soften the onions, celery and garlic in the same oil, adding more if necessary. When they are well softened, stir in the tomato purée, then pour over the red wine, and let it bubble for 5 minutes.
Add the ox cheek and push it well down, then pour in enough stock to barely cover the meat – it should not be too wet. At this point you may want to throw in a bit more flour – not much – to keep the sauce the consistency of thin cream.
Season with more salt and pepper, the orange peel and thyme, cover and bring to a very slow simmer on the top of the stove. Let it barely simmer for 10 minutes and then transfer it to the oven and forget it for 3 hours. Test the meat with a knife-point, it should now be meltingly tender and bathed in rich, dark gravy. If it feels tough, turn the pieces of meat over and continue cooking for another hour. You can make this a day ahead – the flavours mellow if allowed to mature.