Remove the ossobuco from the fridge approx. half an hour before you're ready to start to allow it to reach the right temperature. Preheat the oven to 200°C (392°F). Peel and chop two onion and the garlic. Peel and dice the carrots and celery. Clean and chop the leek. Put some oil in a roasting pan and fry the slices of ossobuco on both sides, then remove. Sweat the chopped onion and garlic in the pan. Add the carrots, celery and leek and brown them. Stir in the tomato purée and then add 500 ml white wine. Add the tomatoes, lemon peel, bay leaf and thyme, then season with salt and pepper. Add the ossobuco slice back into the pan then cover and cook for around 40 minutes in the preheated oven.
In the meantime, make the risotto by peeling and finely chopping the remaining onion. Melt a little butter in a saucepan and then soften the onions in it. Then add rice and saffron and soften. Add 150 ml white wine to the mixture. Cover the pan and turn down the heat to low. Leave for around 20 minutes. Then stir the Parmesan into the risotto. Wash, dry and finely chop the flat parsley.
Serve the ossobuco with the vegetables and sauce, garnished with the parsley. Serve the saffron rice separately.