Heat the olive oil (2 tbsp) and butter (1 tbsp) in a roasting tin over a high heat.
Fry the veal shank slices for 3 minutes on each side until golden brown, then remove them from the pan.
Reduce the heat and sweat the onion, carrot and celery in the cooking fat for 6 minutes.
Add the garlic and tomato purée (1 tbsp) and fry for 1 minute.
Deglaze with white wine (150 ml) and reduce by half.
Add the veal stock (400 ml), bay leaf (1), thyme (3 sprigs) and lemon zest (1 strip).
Return the knuckles to the roasting tin; the stock should come up to halfway up the meat.
Cover the roasting tin with the lid and braise in the oven for 2 hours; after 1 hour, turn the slices over and baste them with the cooking juices.
About 30 minutes before the end of the braising time, heat the vegetable stock (600 ml) in a small saucepan and keep it warm.
Soak the saffron threads (1 pinch) in 3 tbsp hot stock.
Melt the butter (1 tbsp) in a risotto pan and fry the shallot for 3 minutes until translucent.
Add the rice (160 g) and fry for 2 minutes, stirring constantly, until it becomes translucent.
Deglaze with white wine (80 ml) and allow it to reduce almost completely.
Add the hot stock a ladleful at a time, stirring constantly, for about 18 minutes in total, until the rice is creamy and still slightly al dente.
Stir in the saffron along with the soaking liquid.
Remove the pan from the hob, stir in the Parmesan (40 g) and cold butter (1 tbsp), and season with salt to taste.
Leave the risotto to rest, covered, for 2 minutes.
Remove the ossobuco from the oven, carefully lift out the shanks, remove the kitchen string and keep them warm.
Reduce the stock in an open roasting tin on the hob for 5 minutes, then season to taste with salt and pepper.