Preheat the oven to 180ºC/gas 4. Remove the veal rack from the fridge and allow to come up to room temperature.
Place a large frying pan over a high heat. Rub the veal all over with oil and a pinch each of sea salt and black pepper. Place it in the pan and sear all over for 3 to 4 minutes, until golden brown.
Transfer the veal to the baking tray and brush the top with the mustard.
Place the porcini in a small bowl, cover with boiling water and leave for a couple of minutes for the mushrooms to rehydrate and soften. Pick the sage leaves.
In a food processor, add the porcini, bread, chestnuts, sage, a pinch of salt and pepper and a decent splash of oil. Pulse until it all comes together to form a coarse paste.
Spoon the chestnut crust onto the veal and pat it down firmly. Place the rack in the oven for 1 hour 10 minutes to 1 hour 20 minutes. Remove from the oven and leave to rest for at least 30 minutes.
While the veal is resting, make the sauce. Squeeze the orange juice into a saucepan and combine with the remaining ingredients, plus any cooking juices from the veal pan. Place over a low heat, stirring, until smooth and combined.
Season the sauce to taste and serve with the veal rack and some steamed winter greens.